If you're not counting, you hit a 12 vs a 2,3,7,8,9,10 or A. V. 4-6, you stand.
If you are counting, your stand versus a 3 at +2, and vs. 2 at +5 (using AOII... it's been awhile though).
If you're playing BJ in a casino and you're not counting, you really need to ask yourself what you're doing there.