I'm a medical marijuana how long do i hold in a puff of smoke ?

Who knows?: This is exactly the problem with "medical" marijuana. Doctors are not trained in the ins and outs of marijuana use. All we can do is "recommend" it. It's not really a prescription. That said, smoking it may not be the safest way to consume it because of possible damage to your lungs. Consider eating it (in cookies or other food) or using a vaporizer. Ask specifics at your local dispensary.

Don't SMOKE it: Be kind to yourself and your lungs and invest in a vaporizer. These are similar to the mechanism of e-cigarettes, and will volatilize the thc and other agents from the plant without burning (and mostly without any smell). The vapor will almost instantly transfer its contents into the bloodstream, and doesn't have to be held any length of time.