Ahh at I see. Well then maybe a 9V (or even 2 in parallel for longer runtime if it becomes an issue) and a 7805 would be more practical. A rechargeable "9V" is usually 7.2V and would be well suited to this, as extra voltage beyond 6-7V just generates more heat with a 7805. Capacities around 250mAH are typical for NiMH 9V's, so I'd make a rough guess and say it would last 1-2 hours, maybe more if it's a very efficient device and isn't transmitting much.
Edit: if jmetal88 is right, and they really do use that much power continuously (I think the USB maximum is 500mA though, but even that would be a lot), you may want to find another way than 9V's. A single AA NiMH stores more energy than one of them.