There's no limit. But be careful about burning yourself out.

How much you can reasonably handle will depend on your experience, willpower, etc. As you practice more, you will find that you can meditate for longer without much trouble.

What you don't want to do is set a goal that is so astronomical that you really have no hope of achieving it (when you're honest with yourself), and end up forsaking the practice altogether. See the link below about calculating resistance.