depends on how u drive. If you doesn't hit pothole or not often drive on uneven roads.. normally your alignment wont run far.. you can try to check your tyre for any uneven wear to tell if alignment is necessary.

like i said no rules defined.. ur driving style and road condition determine the frequency required.. myself i do it 20k sometime only minor adjustment required. Monitor your tyre wear from time to time at least that gives you the clue