I think its after 5 seconds or something like that. To be completely honest, I find holding in my hits to not do much than just inhaling and exhaling slowly. I probably hold them for 1-2 seconds max and never felt a difference, other than in my lungs lol. I think that the whole "hold it in thing" just helped people figure out/understand how to smoke weed, making it seem like the idea is to take a deeep breath, allow it to fill your lungs, then exhale.