Add it to your secondary. If you don't use a secondary, add it two weeks into primary and then leave it in the primary for another week but no longer. Never add it during the boil as it will ferment out. The closer to bottling you add it, the more maple flavor will remain. Generally with things like maple syrup or honey they tend to ferment out.
As for how much, I'd go with between 8 and 12 ounces. A lot of people use grade B. I always use grade A because it's more expensive. I'm no expert in maple syrup so my perception of it is that if it says 100% pure, comes in a fancy glass bottle, and costs more than Aunt Jemima's then it's probably good.
Experiment. Just don't experiment by adding maple syrup to the boil because it's a lost cause.
Also, always use maple syrup from a sealed bottle. If you're just trying to use the rest of the maple syrup in the pantry you'll have to pasteurize it by holding at a certain heat.