I think it should not take you more than 5 hours per week to exercise (not backed up by science so please look it up!) – but I’m sure that over- exercising is just as bad for your body as not exercising enough!
Start with however long you’re comfortable with, then gradually increase you exercise session. The kind of exercise you do also matters – think of walking, which does not consume as much energy as running for the same period of time.
Lastly, you know your body more than anyone does – people have different needs and different energy levels. Therefore, start small and slowly increase the frequency/intensity of you exercising sessions, but make sure you’re not pushing yourself too hard. Also do keep an eye on how you eat – an appropriate diet plays a pivotal role in staying in shape.