How often should you meditate? The short answer is every day, and don't overdo it.
The long answer: If, like me, you want a lasting improvement in your day-to-day affairs, you should prioritize application and regularity, while evaluating your practice critically, and if possible, taking a course in meditation.
After you finish sitting, can you keep on applying what you've learned for the next 5 minutes? For the next 10 minutes? For the next 10 hours, even? This is something that comes with practice, and moreso if your practice is regular.
R.Barzell's comment about going about your day mindfully is important, because meditation is worth more if you take it as a training on how to handle daily life. When someone says something that used to offend you, if you can look at that the same way you look at your breath, that's a huge benefit. On the other hand you might make many beginner's mistakes, such as thinking "Meditation is an escape from reality that will make me feel good" or being overconfident and smug about your supposed progress. Both are mistakes I have made in the past.
Regularity will make or break your practice. Even if you only sit 10 minutes, you'll get huge benefits from practicing every day, regardless of which tradition you practice (assuming that you stick to a tradition and don't make a random fruit basket of traditions). So if you're sitting for 30 minutes a day every day, you should ideally make more progress than someone who only sits 10 minutes a day every day. But all of this probably doesn't matter if you can't evaluate your practice critically.
This means asking the right questions about your practice.
- What was gained from this practice? What was lost?
- Does this teacher have good qualities? Bad qualities?
- Is there more or less suffering as a result of the practice?
- Am I more prone to distraction or more focused?
To be clear, I don't mean you should to overly analyze this in the mind as our logical abilities can only take us so far. What I mean is to keep an eye out. Try things and see how it goes. As some say, check yourself before you wreck yourself (I really like that expression, hah).
This one probably varies depending on what tradition you practice. In the Mahasi Sayadaw school, any experience that arises should be the main object of meditation. This goes for sleepiness, so if you're sleepy, focus on that. Either you'll fall asleep or the sleepiness will go away. We shouldn't worry about falling asleep, but, in the case that we start worrying, then we should meditate on the worrying itself.
Take a Course
Finally, there's great value in taking a course under an accomplished teacher. A teacher can correct your practice when it goes astray—I know my practice went astray plenty of times, so I'm grateful to my teacher for helping with that. On a course you will probably meditate during every waking hour for many days consecutively. While this may seem to oppose the idea of meditating 10 minutes a day every day, think how much more you can gain from a consistent practice, when you have under your belt lessons learned from 50 or so hours of nearly continuous meditation under the guidance of a teacher.
Beware the Lotus
I forgot to add, be careful about sitting with one leg on top of the other, or even worse, both legs on top of each other. If you don't have the requisite flexibility in your hip sockets, you will do long-term damage to your knees as they bend in ways they are not built to handle. Much less strenuous is Burmese position, where one leg is in front of the other.