would depend on many things. assuming you weren't trying to lose weight, but were trying to work out for longer to increase endurance a lot then its not too much of a big deal if you want to do more. the problem is the body WILL eat its own muscle aswell as energy stores, sugar and fat. if you aren't trying to lose weight you can simply eat more carbs before you exercise and plenty of protein after, and some before would help too, without too many issues. though endurance will only increase if you don't overtrain so one harder session a week or something with a couple days break after it.
if you are trying to lose weight then, assuming you're on a calorie deficient diet aswell, then lack of carbs to burn means muscle loss can become a problem. eating higher levels or protein to compensate is one thing you could do. after 15-20 minutes you would start to burn a lot of fat, first 15 minutes its more carbs you're burning so in theory the longer you can go after that 15 minutes the better for fat loss and i don't think it will be a problem to exercise longer or harder. interval training is pretty good for burning off fat. again a bit of protein before and after will help replace any protein you've lost.
if you are trying to lose weight then putting on muscle will help you a lot. i'd say if you were doing mainly cardio, do first day of the week with less cardio and throw in some compound weights lifting, squats, deadlifts, bench presses and the like will help give core strength and more muscle. eat a crapload of protein after. 2nd session increase that cardio , then a longer harder session with interval training and as it will go longer and harder and you'll burn off a little muscle hard cardio might not go that well with weight training so leave any weights off on days you do really really long cardio sessions.
compound muscle exercises burn a LOT of calories in and of itself because you're working a lot of muscles. but muscle burns energy, every pound of muscle is it, burns 100 cals a day. so the more muscle you can put on the more energy you can burn will sitting on your behind, and it also means more muscle being worked during exercises so you can do more work all the time, its a win win situation. don't forget weight training as part of a good balanced weight loss program. compound lifts also help all the stabiliser muscles and a lot of extra core strength you won't really build doing cardio, but that core strength will make you far less likely to get injured while doing rowing/running/biking, so it just helps in so many ways.
