Well… I must admit that when I get onto a really good game I have played for well over an hour! But it is recommended that you take regular breaks – at least every hour or so. This not only gives your eyes a break, but you will probably need some food and a drink! It is difficult though with many games today, especially the multiplayer online games like World of Warcraft, where is it hard to have regular breaks.
I really would try to have an eye break every 15 mins or so – this can just be as simple as looking out of a window for a few minutes. This depth of field is much more relaxing for our eyes, as they do not have to contract to focus. Bit like an eye stretch really. And make sure you get some fresh air and a bit of exercise – you may find that you are better at your game after a break!
Besides Tess’ very comprehensive answer, I would consider how playing games consoles’ influences other activities. It might be for instance that someone only plays for 45 mins but does so 3 times-a-day, 5 days of the week. It’s probably not just the length of time that influences certain outcomes but the frequency too! If you find that you “have” to play, get irritated if you can’t, and are finding that you’re having less fun doing other things that you’ve previously enjoyed, then those could be signs that you’re is playing it a little too much!