Usually when I'm doing an extract batch, I'll add after steeping specialty grains (if any) and before it actually starts boiling. That way I get it dissolved before the pot gets too hot. Helps to avoid extract burning onto the bottom of the pot.
But, there's no reason to avoid adding some of it later (like at 30), and some advantages like lighter color and more bitterness extracted from the hops.
Personally, I boil the whole thing for 60 minutes.