I have to repeat here that it's really difficult to advise you because your tank is cycling and you don't have any way of testing to see how quickly the water is getting soiled. Until the tanks have established bacterial colonies. . . you'll want to do daily changes of at least 50%, I'd think?
BUT I'm really NOT sure! I keep hoping that someone with more experience will show up to give you better direction on this. I've never cycled or maintained a tank of that size with a frog. You might want to ask on the betta forum. They often have smaller tanks, and frogs, and someone there may be able to point you in the right direction. I don't want to steer you wrong, and I'm new at this, too.
You really should make your next purchase a liquid test kit, though. I can't stress how important this is. . .