I rarely lift for more than 45 minutes. And this is multiple body parts. If I am lifting for more than 45min, I know I am spending too much time sitting and not enough working.
For example: (all times are approximate)
Chest, Shoulders, Triceps - 40-45min
Back, Biceps - 25-30min
Chest, Triceps - 25min
Shoulders, Triceps - 25min
Chest, Back - 35min
Back, Shoulders - 35min
Biceps, Triceps - 20min