I have seen air brake equipped buses keep their air pressure up for several days. They were the exception.
Most newer buses will keep their air pressure up overnight.
But a lot of older buses are hard pressed to keep their air pressure up for four hours.
A lot of the times the leaks that cause the draw down are so little it is hard to locate them. Sometimes all it takes is to tighten up a fitting.
What happens is the more air operated devices you have the more opportunities there are for air to leak. A bus with only air brakes and nothing else has a minimum of air lines and fittings. When you start to add air operated wipers, service door, rear suspension, front and rear suspension, front/drive/tag axle air suspension, driver's seat, horn, air locks on luggage doors, air throttle, air clutch, air transmission shifter, air operated damper doors in the HVAC system, etc. you end up with miles of air lines and forty-leven fittings. It becomes an impossible task to identify and repair every leak.
As long as you can build air pressure up in the required amount of time and it doesn't leak down in less than the required amount of time I wouldn't worry about it.
As it has been noted before, I would not operated the radio with the key in the run position. Outside of the fact you would have to listen to the buzzers going it can also cause issues with the brain boxes for the engine and transmission if the key is left on for extended periods of time with the engine off. It would be much easier to just get the hot lead to the radio connected to an always hot power point.