We've got to do some physical conditioning in space to stave off muscle atrophy and skeletal deterioration caused by less than standard gravity and the recommended daily maximum is an hour. Much more than that can result in small muscle tears which worsen over time and can permanently damage muscle tissue. Not to mention it does a number on the heart and can lead to early onset cardiac arrest.
I mean, I'm a fuckin' big dude and I didn't get this way by not working out, but even I don't push it more than an hour a day or a couple hours every other day.