Add it in after Primary fermentation is complete then let it sit as long as you would normally, bottle or keg, condition, then drink.
If you plan on using a secondary, put it in your secondary vessel and let it sit a few weeks. I typically add vanilla to my primary after about a week and then let it sit for 2 more. Then I bottle and let it condition in the bottle for 3 weeks. I shy away from secondaries unles using fruit or sometimes I use them for dry hopping.