A single decade seems like an unrealistically optimistic best case scenario for the UK at this point. More likely it will take a decade until the negotiations could start at the earliest after the UK completes some desperately needed reforms of its own political and election system.
I think the main issue is UK won't accept they'll be treated like everybody else in the union. There is still an expectation of privilege that is severely misplaced in the context of what has been going on.
If none of this was an issue, I am sure EU would be happy to quickly let them rejoin. Having the UK in the EU strengthens Europe's position, they have shown to defend Democratic values in Ukraine and, once they've come to grips with the end of their exceptionalism on the world stage, they'll be a valuable asset in the world's attempts to progress.
Coming to grips with that as a nation will be painful though, so I expect it to take a while unless something drastic happens.
No, the main issue is that the EU won't let an untrustworthy country like the UK rejoin since it would slow down any action done by the EU. The UK has to prove it is truthworthy again and that don't happen over night.
I think my question would be "why would the EU take us back at this point?" We had until the end of 2020 to hit the "undo" key, but didn't and now the ship has sailed.
The EU has obviously an interest in getting the UK back in some shape. But I would say that nobody is in a hurry right now. The UK needs to sort itself out, decide what it wants and be consequent and solid about it. There are currently many discussions on the shape and reform of the EU and more people defend the idea of different levels of integration available. From a true federation with single currency in its core, to contries with free border, free movement, common market, but own currency and non-federated. That second tier could be ideal for countries like the UK and Ukraine, at least in the foreseable future.
I think it would benefit both, but the UK needs to decide what it wants. The rest of the EU27 already said that the door remains open if the UK changes its mind. It'll be through the regular process, with tough negotiations, but the EU is always open for business and always has been.