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.

This then drops rapidly.

When to sell gets difficult.

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.