The correct time to use the trollface meme is when you have trolled someone or when you are making a trollege(if you don't know what that is Google it). Or when you are just making a meme about someone being trolled or you trolled someone.
The correct time to use memes in general is almost anytime like When you make a meme or when you wanna make something funny. Of course there are time not to use memes, like when someone died irl or at a funeral or just when someone is having a hard time and doesn't want to talk to people. Memes are funny, but only when people want to be entertained.