Bias frames are generally good for some time. Darks can also be reused if you have a temperature controlled camera. Flats are normally redone each session because they are most likely to change (new dust spots, differences in illumination because of camera rotation, etc.)
Rick is correct but sometimes I mess up my flats or they don't correct properly and I have used those from the previous night but only as I hadn't changed the filter ie only used the luminance from night to night and the same focus. So you can get away with it but only rarely. Darks and biases cam be reused for about a month as long as nothing really changes.
If your setup does not get moved or diassembled then you can use the same flats for months on end. That is what I do and i don't have any calibration problems.
My camera stays on the scope. I use darks for about a year and have never made new bias framed. I made a new set and tested it by subtracting the new one from the old one. There was no difference.
Flats last til I take the camera off the scope. You do need different flats for each filter though.
