If the window is closed, your cham isn't getting UVB since it does not go through glass. I let my guys hang out in the window all day long if they so desire. If I have a day where I can actually open the window, they can bask all day if they want to. There's really no such thing as over exposure, they'll move out of it when they want to. If your cham gets a lot of natural UVB (like daily, for a couple hours or more) then you'd need to adjust your calcium with D3 supplementing some.