I've finally got my first stock with my employers through their discount purchase plan. Does anyone have anecdotes or interesting math on when to sell?
It depends on the rules of the plan. Some plans require you hold for a certain amount of time, while others may let you sell immediately.
When I had employee stock, my philosophy was to sell ASAP and treat the discount as a cash bonus. Since I did not have an allocation for single company stocks in my investment plan, it made the most sense to sell and reinvest that money into VTSAX.
One item I think interesting, is that the effective period for the initial gain is one-half of the offer period. A 15% gain with a 6 month offer period becomes a 60% APR.