Usual routine is to pump up to your comfort level vacuum, maintain that for 10 to 20 minutes, reduce the vacuum to zero, and remove your cock - the session is over. Normally, you'd do this routine about 3 times per week. Longer sessions or more sessions per week runs risk of localized swelling, usually around the base of the glans if you're cut or puffy swelling of the foreskin if you're not. Also, vacuum greater than about 20 mm Hg (milli-meters of mercury) may cause burst capillaries and bruising.