Jetwings,
We are subject to INAC's (Portugal CAA) FTLs. So yes, we do have limits! The minimum rest is 11 hours. There's other bits and pieces but it depends on when you started, how many sectors, how many time zones you crossed etc. 55 hours per tour. Note, your airline to the aircraft does not count for the 55 hours but (now) it (Day 1) does influence Day 2. It is a lot better than it used to be.
It's rare for me (1 of 1,000) to do much more than 10 hours duty these days, although, 12 hour duty isn't uncommon in Summer.
Again, depending on your day and the hotel, you *should* be able to get to the gym at least once per tour. It does also depend on your motivation.
You do get tired (assuming not crossing a lot of time zones) for sure, but sometimes you do just have to say "no" to meeting up with your colleagues or hit the sack early.