When to use Flare?

I was wondering when should I be using Flare, should I use it when there's more than 3 mobs?

Always and for everything.

^
Agreed.

When MP is not below certain threshold (the lowest amounts of MP allowed to use Flare) and you see more than single target

Are there at least two enemies?
Are you in Astral Fire?

If both are yes, use Flare.

all the above, plus
for single target, if you have convert up, do Fire 3 > Fire1 till around 300mp > Flare > convert > Fire 1 > ice 3.

also, if you know the boss will jump , (i.e. ex primal) just Swiftcast > Flare then transpose. you will have downtime after they jump anyway.

make sure you Swift cast > Flare

very much agreed. Will also say at all cost try to avoid to flare after a transpose.
As you do larger group of enemies you can then be doing triple flares :D

Should I make sure that I can use convert after a flare? And how would I triple flare @Maero?

Oh yeah, and don't use Fire 2. Like, ever. As soon as you have Flare, it is obsolete.

Learn the double Flare rotation. Love the double Flare rotation. Be the best you you can be.

Please correct me if I'm wrong, isn't it better to stick to a single-target rotation if there are only 2 mobs?

Another time you can use flare is at the end of your single-target rotation if you have swiftcast & convert up. Of course you only want to do this if you don't need the swiftcast & convert up later for certain DPS checks.

I wouldn't say never use Fire 2, because some tanks simply can't handle Flare spam, unfortunately. You may have to use Fire 2's in situations where the tank can't handle you generating hate as quickly as you will using Flare spam.