The test before your meal is supposed to be like a fasting test, you shouldn't really snack just before or drink anything if it is likely to affect your Bg levels. If you want to get a truly 'accurate' pre meal level then you have to stop the snacks and munchies just before, same with drinks etc. There is no set time limit, if it is likely to affect the reading because of the carb content then don't do it.
I don't have that problem because I don't usually eat between meals, certainly not just before anyway.
Ken