I have never, ever seen my juvenile bullfrog, Gnag, full. She's always been eager for more crickets and earthworms, despite the fact I feed her 5-8 each night, earthworms occasionally. The same goes for my Green frog. Gnag's a bit of a fat frog, but I keep an eye on her weight. It's hard to tell whether they are overweight while they are sitting, because their legs are pressed against their body, making them appear to be fat. From my experience, the best way to judge if a frog is overweight is to look at them when they are swimming, or simply weighing them. Since both Gnag and Bumpy are relatively skittish, I can only judge their weight by looking at them when they are out and hunting by the lily pads. However, this works fine for me, since the water is always very clear. Also, I can check for any abnormalities at this time too, such as red spots or bumps.

For my Chorus frog, which is slightly more similar to a White's than a bullfrog, I check his weight by simply watching him swim in his water bowl. Despite the fact I mist his environment 3 times a day, he still likes to go into his water bowl at night occasionally, and stare at crickets from the waterline before leaping at them. Unlike my bullfrog, he'll only eat a couple of crickets and then be done with it, and he will only hunt in darkness. He's very similar to my salamanders in this aspect, whilst my bullfrog and green frog will hunt in the evening. So, yeah, Michael is right. It truly does depend on the frog. However, I find that some species, mostly the larger ones, will take food faster than others.