Ussually at least two minutes on heavy compound lifts, around 1 to 1 1/2 on concentrated lifts. Really it's about like everyone else has said, when you feel you can hit it at 110%.
I actually have been using a lifting log on my Handspring Visor to keep up with what I am doing in the gym. It keeps it all up, will tell me my intensity level, when to increase to what weight on an intensity setting, and graph it all out. It also keeps track of lift times and beeps when it is time. I am getting real f'ing lazy. The Palm platform is turning my brain to mush, I have to remember nothing.