I don't have much time during the week to work out so how many hours should I work out on the weekend?

If you only have two days to work out, I would suggest a 'push' and 'pull' day, you do all the pushing exercises (Chest-presses, shoulder-presses, squats) on one day and just don't do too many exercises for each group. And on pull day (Biceps, back, hamstrings) do the same thing. It's not an ideal routine, but it works best since you don't have much time to dedicate to weights.