1. Always prep the pad with a spray or two of QD.
2. So hard to say, you basically work the polish until it breaks down. With some it takes ages (or if you put a lot on), some break down quickly if the speed is too high, but really I'd expect to use about 4-6 goes on a bonnet. You're doing it right though, it's better to use multiple passes than go all-in and wreck the paint. Not that you really can with a DAS, but you know what I mean.
3. When they're falling apart, bin them. Otherwise, don't worry too much.