Haha well, depending on the size of your frog you'll have to get a feel for how much she can gobble down.
If you're feeding her say, every 3 days, 3 cockroaches should suffice. Granted I don't know her size, but for an adult if the cockroachs are fairly good size 3 should do, maybe 4.
So if you're feeding her on a 3 day schedule, I would say every 9 days to dust your feeders.
Also with the mouse thing; they make a great snack for them. However, they are junk food to them as our candy bars our to us. They taste great, but they lack the nutrients needed to support a healthy body. So if/when you do feed your frog mice, I would say no often than one every 2 weeks. Also you could dust the mouse itself, if you feel your frog needs it.