I have a foot tall mango plant in a 8inch diameter pot. Not sure when should I move the plant into a one size up pot?
It depends, if there is still place for the roots then it is fine. If your plant becomes root bound then it is definitely transplanting time. If you do repot a too small Plant into a bigger pot, then it will grow fine too.
Where are you situated @Outgoing? I can't see any information on your profile. If you have winter at the moment like we do, it might be better to wait until spring when the weather warms up and everything starts growing again.
I am in New South Wales Australia, late summer here. Yes, the second year of my mango tree, suddenly grows quick.
