Agreed Preacher...I like to see a lil build up to the action too
The answer to your question though kinda varies depending on the niche. If it's hardcore or some variant on hardcore (blowjobs, dildo fucking, anal, etc.), I like to see some action starting no later than half way through the pics.
Cumshots and facials are a bit different to me in that you can't really expect some poor bastard to cum through too many pics. He'd end up looking like a decicated mummy or something
So, for the cumshots niche...I usually go with at least the last 5 to 7 pics having some goo in em. This can usually be accomplished without getting too repetitive in the pics. Along with some good hardcore shots leading up the the cumshot of course.