When I trained for marathons and triathlons, I exercised 6 days a week, for 20-40 hours a week depending on the season. It's not too much if you're smart about it.
Now I still do 7 days a week, but it's because I have a daily yoga practice and do pilates 2-3x a week. My family and I hike at least 1-2x a week. I jog and swim occasionally.
It depends on what exercises you do on which day. Don't work the same muscle group two consecutive days in a row. If your lifting, you really need the rest day between working a group allow your muscle cells to repair and grow. That rest time is crucial.
Your body will tell you if it's too much for you. Know the difference between soreness and injury. If you feel too tired, or get frequent colds, or get stress injuries, then you need to pull back... but ideally, don't push yourself to the point.