The inference of your statement is that a gap should be left between treatments, presumably for a good reason, ie if you have the treatments too close together it can cause a problem. The focus of the question is on that gap being of adequate length, and if you don't mention the gap it may not make sense. If you just say "how long do I need between treatments?" it raises the question "need for what?" Does something specific have to be done between treatments? If it is just a waiting period, then that needs to be referenced.
If your reason for wanting to omit these words is to shorten the sentence, one alternative would be:
How long should I wait between laser hair removal treatments?