My understanding is that you lose the 15% benefit if you sell before 2 years of joining the program. Say you join this week, you would setup monthly salary deductions that would go towards IBM stock, just like 401k deposits. The earliest you could sell is April 2024, no matter how many buys you made.

Additionally for tax purposes, you need to hold a stock for 1 year and 1 day for it to be taxed as long term gains, which is a much smaller tax. So say you sign up for stock purchase program this week. You first purchase would be April. You keep buying monthly until October 2023. So, on the IBM side your trigger starts on April 2022 and you can still sell April 2024 while keeping 15% discount, however to realize long term tax benefits you would need to hold until October 2024 since thats 1 year after the last purchase.