This will depend on a number of factors, but generally speaking you should allow around 20 to 30 minutes between application and sanding.
Some of the factors that will affect this include the product you have actually selected, with some brands of car body filer drying a little faster (or slower) than some others). The thickness of the filler will also of course have an impact on drying time, so allow more time for drying when repairing deeper dents and even holes.
Lastly weather conditions can play a part too. If you are working out in the open in the middle of summer, the drying time is going to be cut back a little.