A week, under normal riding conditions. With the BMX, you'll probably want lower pressure for riding in the sand, and higher for hard dirt/paved roads. In that case, replacing air is required before every ride. If you can judge air pressure by touch, that makes a quick pre-ride check easy, but you should always check PSI before every ride... assuming your tires are rated 40 - 60 PSI, in a week you'll probably only drop from 60 to 50+/-, and that is a good point to top off. Otherwise, they'll stay in the safe zone for weeks with harder pedal power required and smoother shock absorption. That is a judgement call for the rider, but either way, check PSI before every ride, and every now and again a slow overnight leak will be found.