When I'm alone I like smoking a nice fat blunt to myself. Either that or smoking 4-5 bowls one after another until I feel the high coming out of my eyes That's when I know I don't need anymore.
With friends, it doesn't matter because the goal is always to get stoned off our asses. A typical night consists of a few J's, Blunts, bong rips, etc... With them, I stop when they stop because I always want to keep smoking more and more!
When I've been holding the bowl and lighter in my hand for 5 minutes staring off into space. At that point I take one more hit (if there is any left) and put it down for awhile. xD