The quick answer is 10, 15, 20, 25, 30, or 40 seconds.
Vapor takes longer to be assimilated, but watch out for oxygen deprivation (maybe take a few quick inhalations if you want to hold one really long, i do)
i hold each hit at least 20 seconds, do quick breaths before, and a little inhale 3-4 seconds before i have to release.
as a rule of thumb, don't pressurize your lungs, hold it in a way that allows for expansion, don't hold it tight
if you experience irritation at the back of your tongue, try clicking it against the back of your throat real quick to kind of wash the condensed vapor off, and keep you from feeling like you have to cough