personally i dont start another set until i feel i can put another 100% into it again.
il usually wait sometimes up to 3-5 minutes before doing another set on DB incline bench depending on the weight. and the last set il usually do a drop set like 6x120 drop to 12x50
while im resting il keep flexing the pecs to keep the blood in them so i dont loose my pump...
my .02, works great for me!