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

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.