when is an appropriate time to sell the stocks your employer gives you?

I’ll have about 20-25L worth of company stocks in the next 5 years. I understand that they can rise and fall every year. Is it wise to hold onto them until retirement or can I sell them after 10-15 years? Remember that there is no diversified portfolio for this, all of it is from the same company! How much of a CAGR can I expect on this (it’s a US based company, well reputed)?