Depends on the person, heh. Some folks can go several hours without really being bothered by it, some can't. Varies based on the individual, their learning style, surrounding environment, and so on. There's not really a single method that's sure to work for any particular studier.
If you're finding you can't manage a straight hour without getting tired, then don't. Break it up, do smaller sessions more often -- ten minutes here, twenty there, etc. Last I checked there's been a fair number of studies showing that trying to study more than an hour or two actually hurts your information retention and comprehension. Marathon studying may be something you actually want to actively avoid.
Beyond that, try different methods of studying. If you're primarily just reading, do other things -- read out loud, record yourself reading out loud (or your teacher, if you have lectures and they allow it) and play back the information later, write it all out (and possibly translate the material from one format to another -- ferex, when I was doing math heavy work, I'd often do the assignment up in a spreadsheet program instead of trying to work it out on lined paper or whathaveyou), get other people involved (if it's an option), and so on. As an example, one of the things I'd do (surroundings permitting) fairly often with longer sessions was hold conversations (out loud) with myself on the subject in question, pacing back and forth and more or less ranting at myself. It helped with certain subjects. Movement and articulation can do wonders to help with subject engagement, for some people.
Push comes to shove, study until you're tired, take a break/nap and then do it again. It's entirely possible that, for you in particular, there isn't a way to not get tired after extended studying.
The big thing with studying (and learning in general) is there's not a single method that's effective with all people trying to learn -- there's many different methods, and each person is going to use and react to those methods in different ways. If you want to see notable improvement in your studying habits, you'll have to experiment and find out the way you learn most effectively. If you have people around you that you can talk to or sit in on their study sessions, do so -- find out what they're doing and what's working for them, and then try it for yourself.