Generally, weight training workouts last about an hour. That includes:
Of course, depending on your fitness level, the intensity of your workout and overall goals, some may last longer. For the average person, an hour will suffice.
Drink lots of water, up to a liter per hour while exercising (weight training or cardio). Going beyond an hour if you are working out very intensely or are a beginner can lead to dehydration and possibly excessive muscle tissue damage.
Stretching after a workout is very important. It helps relieve the tension in the worked muscle area and promotes better all-around flexibility and mobility. Not stretching after a workout can lead to muscle stiffness and excessive soreness after a workout, which over time can lead to injury.