I think the best time to sell your altcoins is during the market bull run - altcoins market bull run to be specific. If you buy altcoins during the ICO/IEO you should be ready to sell them once they got listed on a major exchange. what way you can make a lot of money.
You are assuming that as soon as a new coin is listed in a major exchange it is going to go up in price but the number of coins that do that is very limited.
Most coins as soon as are listed in an exchange go down in price immediately as investors begin to try to find buyers for their coins and then they realize there is no demand for them, when they realize this they begin to panic and as a result of this the price of their coins begins to go down even more.