It kind of depends on the specific bearded dragon, how much they eat, and how often they eat. My bearded dragon (who is a little over six months old now) goes once every day. Typically, younger bearded dragons will go daily, some twice a day. As they get older, they may go less often. If you notice they are not defecating regularly according to their own personal schedule, then there could be a problem. More than a few days without defecating (associated with other symptoms, such as lack of appetite or energy, etc.), and anything longer than a week may indicate some problems.
Once a day is fine. Now that you get a good feel for their defecating habits, it's usually a good way to gauge how they are feeling. If it changes drastically, it could be an indicator that something is up. But right now, that's perfectly normal, and hopefully it's consistent (until the get older, in which when they are well over a year in age, they'll most likely start defecating less often).