As your interest in meditation grows, you'll find yourself making more room in your schedule for practice. Seasoned meditators manage three or four hours of practice a day. And they enjoy it. It comes naturally. Sit as long as you can, but don't overdo.
Most beginners start with twenty or thirty minutes.
Initially, it's difficult to sit longer than that with profit. The posture is unfamiliar to Westerners, and it takes a bit of time for the body to adjust. The mental skills are equally unfamiliar, and that adjustment takes time, too. As you grow accustomed to procedure, you can extend your meditation little by little. We recommend that after a year or so of steady practice you should be sitting comfortable for an hour at a time.