I know ICO's won't sell all the generated tokens during their sale. They will keep some for internal distribution like for Teams, Advisors,... Once the sale is done investors can trade tokens but internally distributed tokens can't be used for trading.

What is the best optimal period to lock tokens internally say for 2 years or 3 years? On what basis ICO's are doing it?

I have seen many ICO's selling tokens for cryptocurrencies. What will they do with those unsold tokens at the end of their ICO? Some say they will burn it so no one can use it. Are they burning it or is it really the good to burn those unsold tokens?