I tapered from 150 to 37.5 of effexor (venlafaxine) and then quit 5 days ago. how long will withdrawal last?

Varies a lot: Most people taper Effexor (venlafaxine) over a week or two, & are fine after that. But many need to go more slowly. If you still have withdrawal (discontinuation syndrome) 5 days after stopping 37.5, then the taper to 37.5 was too fast, or you're especially sensitive to discontinuation. Talk to your prescriber about continuing 37.5 a few more weeks, & maybe taper further by dividing those 37.5s.