Depending on what you use for bedding, a month is about what I do. For aspen bedding you can spot clean. Scoop out the poo and wet area when they go. For news paper or brown packing paper, change it when they go. As for the top. They actually do not need as much ventilation as you would think. Many of us that use a reptile rack with tubs do not have holes in the tubs. The only ventilation is the tiny gap between the top edge of the tub and the shelf it slides into. You can cover most of the top and only leave a couple inches or so on one side for ventilation. If you are using a towel or something cloth, you dont even need to leave a gap since fabric breathes.