when is it time to move on?

Im experience locked to stay at level 90 and my ilvl is 500 currently.
I just ran a normal MoP dungeon and according to Details there were a number of times I my DPS would be double, triple or even quadruple the next highest.
its showing that I can run heroic, but I never know when I should because on my last character I did it immediately and was a serious dead weight on the team.
Is there some magic ilvl number I should be looking at before moving on to Herioc MoP dungeons?

Any help appreciated.

I didn't find the right solution from the Internet.

reference:
us.battle.net/forums/en/wow/topic/20761847877
Animation Production Cost

Thanks!