There's no fixed rule. It depends on the size of the plant relative to the size of the pot, as well as the humidity levels. They like higher humidity, so if you haven't done so already, try a pebble tray. Water when the mix is almost dry.

I only have to water my rhizo 'Tiger Kitten' about once every two weeks. Its in a four inch pot. Doesn't get dry before that.

Randi

well, in keeping with what elizabeth is saying, there is no fixed rule. if your potting mix is lighter than randi's, you may be watering more often. go with the pebble dish, and stick your finger in the soil to help determine when you should water--let dry between waterings (overwatering is a major source of begonia death). the crisping leaf edges are, as mentioned, probably due to humiditiy being too low.

good luck.
-ming