It's a very complex question, but I'll try to summarize and give general principles.
If you use your Flash (for whatever reason), it means you won't have Flash available in the next 5 minutes. Obvious, right? So, the question you have to ask yourself before using Flash is: "Will I need Flash in the next 5 minutes for something more important than what I want to do right now?" You can't be sure, of course, but you have to guess.
As an ADC in late game, for instance, using Flash to chase that isolated player means you probably won't have it back for the next team fight in 2 or 3 minutes. If their team jumps on you and insta-kill you at the beginning of the fight, your team will probably lose the fight horribly, and this is obviously much more important than getting one kill.
Another opposite example: you're Renekton Top lane, it's 5 minutes into the game. Should you Flash to get First Blood? Well, probably. Getting First Blood means you'll be able to have an item advantage and dominate your lane, so you won't need your Flash defensively against your laner. You will need to be careful about the enemy Jungler, since you won't have Flash to escape ganks, but with the kill gold, you can buy a pink ward and put it in the river. So yes, using Flash is probably a good idea.
(As a side note, if you're interested in Economics, that's called an opportunity cost, you can read a bit more about it on Wikipedia or something like that.)