It really depends on the flake as to drying time. Some flakes, like Solani 633, is pretty much ready to go out of the tin but something like Full Virginia Flake will need a lot of drying time. I have a halogen lamp for another hobby and will place the tobacco on a tray under that. It puts off a decent amount of heat so it dries the tobacco rather quickly. Still, FVF or St. James Flake can take a good 20 minutes under the lamp.
I find it better to rub out before drying as once the flake is dry, it will not rub out well. You could prepare the whole tin, I typically will do enough to fill three to five bowls at a time. It will dry faster if rubbed out in my experience.