recently i hired a barbell / weight coach to train me (he really pushes me hard, harder than i ever thought i was capable of going), i find that i usually start wanting to mentally 'give up' at around the halfway mark during a training session. 40-60%.

but since i'm generally healthy, the trick is to decide to go until i absolutely can not move anymore, whether it's 2 minutes or another hour. once you've made that decision in your head (your mental "escape route" having been established), the rest is easy. when you have a professional watching your form and supervising you, this is perfectly safe. if i can still move the weight, i keep going. if i can't move the weight, then it's a good time to stop.

for me personally, it's much harder to wake up 3x a week at 0530 and go to the gym than to finish a physical task when you feel like giving up. that's where the 40% rule doesn't really help me much. once i'm there it's not that hard to finish.

btw, for those of you thinking of hiring a trainer, don't just walk into a gym and ask for one on staff, find a reputable barbell coach in your area and work with them on a long term training plan that suits your budget and goals, then pay it all up front because otherwise you're not going to get your ass out of bed at 6am.