How long you can expose without trails depends on the lens used and where it is pointed. The sky's rotation is fastest near the the celestial equator (0 degrees declination) and least near the pole. Its rate decreases with the cosine of the declination.

Simply take a test photo and see what you get. Back in film days I'd have worried about things like this as film was expensive but in this digital age it costs nothing to do a lot of experimenting. I'd start around 15 seconds. It all depends on what you can tolerate.

I built first a simple single hinge drive for my 50mm lens with 35mm film that worked very well for film shots up to 10 with me turning the screw a quarter turn every 15 seconds. I later built a double arm version hitched to a circa 1930 wind up alarm clock. It drove lenses up to 400mm very well up to 30 minutes though I used a small spotting scope with cross hair eyepiece for a guider so the set up was complicated but all I could afford as a college student working my way through school nearly 50 years ago.

Today there are stacking programs that can stack a series of short exposures adjusting for the rotation allowing the equivalent of much longer exposures. Not as good a a single exposure of the same total time but sure a big improvement just the same. DeepSkyStacker is a free program that appears able to do this. If you use one of the reject methods it would eliminate planes and satellites at the same time.

Simple star trail shots can be quite interesting as well. If light pollution is really bad you can again stack a series of short exposures this time without alignment to create star trail shots that would be destroyed by light pollution prior to the digital age.

Rick