I got trolled pretty heavily for saying this in another thread. But my experience, and that of the brewers that I learned from (many who are BJCP judges, own/run homebrew shops, and publish the local craft/homebrewing magazine) believe that the rule of thumb for tripels is that they benefit from 6 months to a year in secondary. Since they're high grav, and have a goodly amount of simple sugars in them, they kind of have a rocket fuel, rawness about them, that bulk conditioning rounds out.
As for priming them, if beer is in secondary for for anything over six months, we recommend you add a little fresh yeast on bottling day along with the priming sugar. It will help.