I try and keep a minimum of 3lbs of special B on hand. But just like my bottles of franks red hot, I look a couple days after getting it and suddenly I am out.
I know it's not a replacement for 120L Crystal, but it is much more flavorful and complex so if I am looking at a recipe and they use 120 I just use special B instead. Sipping s milk stout right now with 4oz of special B (I think it's like 2-3% of the malt bill) that's quite enjoyable. I think you'd have to hit the 7-10% range for it to be very assertive in a stout(this doesn't include American/dry stouts).