There's not really a magic number. It has enough bulk to shrug off weak attacks from, say, wall oriented mons, but without full attack investment it can't really do that much back to them either. Max attack is pretty much always recommended because you want to constantly be dealing the most damage you can as playing Pidgeot as a hit and runner is optimal for it in OU.
You simply can't sacrifice speed, so you'd be splitting between bulk and attack when even a fully invested defense doesn't do that much for it. The best defensive purpose for Pidgeot are its immunities likes ghost and ground where Pidgeot doesn't need investment defensively anyways.
At the end of the day, you want to keep the bird's sp.attack EV's in the 200+. At which point, is a few dozen bulk points really going to be making a difference against mons that won't hit it hard to begin with? The switches Pidgeot forces with sub are going to be mons you want to hit really hard anyways, so bulk gets a little redundant.
You can experiment with featherdance, toxic, work up, etc. A bulky work up set could potentially do real work, but for the most part the main birdspam set is what works best in OU's environment without needing a crazy amount of support. If you want to experiment with bulky / unique sets I would try playing UU (as well as RU where Pidgeot is currently legal but probably won't be for long) where the metagames are much, much more forgiving for it.