Meditation is recovery: a recovery period from the strenuous activities of thinking and acting and doing. You'll find that if you meditate extensively, even your need for sleep drops to a minimum, because neither your body nor your mind needs down-time.
Of course, if you meditate from a place of achievement, as though you are trying to accomplish something specific and are thus training yourself to be a superior meditator... Well, that can be exhausting in its own right, though it is a stage all of us go through on the path.
Don't be obsessive about meditation. Daily meditation is good, but putting more than an hour a day into meditation is probably excessive (unless you are specifically intending to go the monastic route). On a three-day, ten-day, or thirty-day retreat you might spend as much as eight hours a day meditating, and that's fine, but you should take breaks for meals, walking, and other bodily necessities. You can — technically speaking — meditate for as long as you like (or rather, for as long as you're able), if you have the proper support system. I'm sure you've heard stories of gurus and monastics who've spent weeks in meditation, being fed and cared for by devotees. But for the most part, you can trust your greater self. Once you've established yourself well enough that you can comfortable meditate for an hour or so, just forget about time. You'll go into the meditative state, and when it's the right time for you to come out of it, you'll come out of it. Big-mind knows better than little-mind what's needed; little-mind just has to come to understand that.