I did a cherry vanilla porter, that I aged in the secondary with 3 vanilla beans for a month and it turned out great. What I'm trying to say is vanilla beans differ greatly from year to year, store to store, etc., kind of like hops. Just go with your taste, take a sample an taste it at one, two, maybe three weeks and determine what you like. When you are happy with the flavor rack the beer off the vanilla. Another great way to boost vanilla taste or aroma is with good quality vanilla extract.