Yeh food drops to the bottom regularly. I feed 6 days a week and leave once with my syrian. This means he finds all the food that falls to the bottom and keeps his hoard low. Less waste and everything is kept fresh
ultimately i wouldnt worry about it though as Ria said.
Yes follow your nose. I had a robo last year and i only took out soiled bedding and added new when it started to deplete. i spot cleaned weekly.
It also depends on the size of the cage. A smaller cage will start to smell much sooner than a larger cage. As well as the type of bedding used.