the "ideal time" is supposed to be 15-17 seconds....and of course you will get more thc if you hold it in longer...no one's ever it any other way....but most of the thc is abosrbed rather quickly so it's just personal prefrence and wastefulness per person that will say how long to hold it....if you're going to be smoking alot i'd say you don't have to hold it in as long (and hell if you are with a bunch of people and don't wanna get ragged on for being weak then you might wanna only hold it for two or three seconds....) but if you're by yourself or only have so much then i'd say hold it in for 15 seconds or more....