Do what feels right. If you're doing an hour a day and that's too much, you'll know soon!
If I over-exercise, my recovery time sucks. Instead of being tired at the end of a workout (normal for me), I'll be tired for the rest of the day, be exhausted at bedtime, wake up tired, and not want to exercise the next day. In the beginning, I found it important to take a rest day every week--to give my body time to recover, but also to myself a "mental break" from exercising.
When I started, I exercised less each day, but my body has definitely become used to exercising longer. I also find it helpful to do different types of exercise--if you should want to try doing more than an hour, try doing some cardio and then strength. Since they're different types of exercise, and are challenging me differently, I can exercise longer.
There are many people here who exercise for longer than an hour a day. I bike to work every day (almost an hour) and then do a 45- or 60-minute class at the gym on top of that. For some people here, an hour a day is their minimum, to stay in shape, or maintain weight loss, or both. However, we've all built up to this point--I certainly didn't exercise for 2 hours a day when I started out!
Basically--listen to your body. See how you feel after your workout, if it starts taking you longer to recover. See if you end up dragging yourself through the rest of the day. See if you're tired all the time, even after waking up. If that's the case, you can vary the type of exercise you do, or the intensity, or both. Or come back and ask us more questions.
