Recognizing a good time to sell a coin depends on some factors which include (but not limited to) the future of the project, technical analyses, future development plans, the current phase of the market cycle, etc

THE FUTURE OF THE PROJECT
For a project that has a bright future, the right time to sell the coin would always be NEVER!. yea, you heard me right! a project with a bright future should be held for the future. Personally, I never get rid of a token unless I feel it would end up being a shit coin. Holding will benefit you in every way. even you can't hold all, hold at least 25%. your future self will thank you for that.

TECHNICAL ANALYSES
Ignoring technical analyses is always a mistake. if you are a good technical analyst, you could wait for a good period when you think the market will fall before selling. you could even decide to buy later if you have a trade setup.

FUTURE DEVELOPMENT PLANS: If the projects have any future development plans and you know they are serious about these plans, it wouldn't be a good idea to sell so soon.