The answer to this depends on individual body composition, genetics, and a drug’s approximate half-life. Effexor has an incredibly short half-life of about 5 hours. Basically, this means that if you took 100mg of Effexor at 10am, you would have about 50mg left in your system by the time 3pm rolled around. At 8pm, you would have about 25mg left in your system, and so on. So Effexor might be completely out of your system in two to three days.