There are way too many factors that go into it. How much you smoke, how long you hold it in, the type of device used, the type of weed. If you kids are only getting high for 20-45 minutes you are smoking some weak shit. It is definitively possible to take one gravity bong hit and be high for 4-5 hours. Gravity bongs (look it up if you don't know what it is. It is definitely not a regular bong)/vaporizers are a totally different thing than a regular bowl or joint.