Need tips, when is a non-campaign game considered 'completed'?

Whenever you’ve had your fill. Whether it’s 5 minutes for 500 hours, there’s no sense wasting your time playing something you don’t enjoy. I’ve played Pavlov for over 300 hours and am still going back every week. There’s no longterm goals and that’s perfectly fine, if I ever get bored with it I’ll uninstall and move on.

When you're no longer having fun.

Just play until you don't feel like it anymore. If you need a milestone, clear a match/run/whatever on the highest difficulty. Or maybe every variety, so in Civ, a victory of each type. Or if you feel like going hard, find a rating you want to get and get there, like Celestial in Strive. I would say just play until you feel satisfied though. Don't worry about what others think of your completion metric. None will satisfy everyone.

Whenever you want it to be

For Civ 6, I'd say winning each victory once. Try to do it with different civs each time too. You can set your goal as winning a game on the highest difficulty if you want, but personally I don't find that to be as interesting as the shift in gameplay necessary to win the different victories without just militarily crushing everyone else.

If the meat of the game is in it's multiplayer, then you just need to be declared as the grand champion.

For something like Civ or Stellaris, I'd count "completion" once I've won at least one game. Because, ideally, I've shown some mastery of knowledge, skills, and mechanics that allowed me to win. I don't need to play and win as each leader in Civ or every race/trait and combo in Stellaris to say I've completed it.

This is similar to how I'd view "completion" in open-ended games like Cities:Skylines or Banished. Having played a city or town for several hours, was I able to keep the residents alive, stabilize the city if there were any issues, and also grow and develop the settlement for a significant, though arbitrary, length of in-game time? If the answer is Yes to all of these, then I've "completed" the game. I've understood how things work in the game. Doesn't mean I have to understand every nuance or know every little trick. But I know enough that things are going well and largely continue to go well. And every time I start a new map, things tend to always go well.

Earlier this year, I stopped playing Eve Online for the nth time after mostly playing straight through since 2019. Because I viewed my time during this last 4-5yr stint as "complete." I achieved practically all the goals I set out to do: join a major alliance, join massive PVP fights, engage in smaller PVP fights, make money that I ever had before, buy and fly ships I'd never used before, learn how to explore and navigate wormholes, try out specific types of industry, play with IRL friends, own and run my/our own station, and more.

In all of these, "completion" obviously doesn't mean I'll never go back. There's always more to do, new things to see. But for now, I am satisfied with my progress, experience, and understanding. I'm no longer a noob.

As someone that played R6 siege for over 6 years. It is complete when the game's dysfunction can no longer be overlooked because of the intermittent highs you get from a good play you made in a round.

Just to add some even longer time goals to the other replies: you could get all achievements for games that have them. Though some of those, like the ones for Civ 6, are excessive. It could give you ideas or shorter term goals to work towards, then you can decide if you've had enough at any point before 100% if things get too BS.

Civilization is like a board game. It's over when you hit the turn limit.

Fighting games usually do have campaigns, of a sort. It's each character's story line. It can be over when you beat every character or if you go through every character's story (ie run through the main mode all the way through with each character you can play as).

Racing games also have campaigns usually, and some even have progression systems. But the campaign is just going through progressively more difficult races. They're complete when you've gotten the best rank on every track and/or collected every part.

They're all meant to be played over and over again though; often against other humans. These are also the types of games that come with a lot of options, settings and modes to facilitate playing the same game slightly differently and keep things fun. So I guess the real answer is: whenever you want. If you're playing then because they're fun they never have to end. If you're playing as a completionist, it's over when you've gotten all the achievements/experienced every piece of content you can access.

Typically, for any game that has a campaign, I would consider completing that completing the game.

That doesn't mean you can't continue to have fun in endless modes or multiplayer. That's a different orientation.

For multiplayer games, there's no completion really. Play the tutorial? All maps once? Win once? Ranks? Endless leveling progress? All achievements? None of those really fit. There is no completion to a game without designed, completable progress. If there's a max level, one could consider that a kind of completion. All achievements may subjectively fit too.

Given that I mostly play heavily-modded games, a run is usually "complete" when it is abandoned due to its inevitable TPS death.

I play a lot of board games. And I own a lot of board games. Not all of my games get played very much, so I like to track each play and over time see which games are forgotten gems or which games I'd be best to just trade away.

In the board game community, you might come across people talking about the "Friendless" metric of their collection. It's a totally made up measurement, invented by a person with the user name Friendless. In that way, it's like the Elo rating in chess and other games. I find it's useful to know when I'm "done" with something that doesn't really have an end, like playing board games. You can always play one more game.

Friendless hypothesized that if you play a game 10 times, you've gained 90% of its remaining utility. So after 10 plays, you consumed 90% of the game play that game provides. After another 10 plays, you're at 99%. By the time you reach 30 plays, you've consumed 99.9% of the game.

You can do the same with games. Maybe the number of plays changes a bit. Maybe it's not the number of plays, but the number of hours. I would say that games of Civ are like games of any other board game: 10 = 90% utility gained. Matches in COD, probably not the same.

As others have said, the best answer is "whenever you want", though obviously sometimes you don't know when that is! If you need a little more structure you can see what the game presents to you as all the things you can do, for instance by completing all the achievements. Remember that sometimes the final achievements are ludicrously difficult, so if you're not enjoying it just call it a day!