I've never actually used maple syrup other than for bottling–it did really well in my rauchbier last summer and my Bourbon Black Ale this winter–but if I were doing it, I would use it like I do honey and dilute it with (boiled, cooled) water and add at the height of fermentation to keep as much of the taste as possible. I've done this with 3 beers so far–my belgian tripel, white house honey ale, and most recently my belgian wit–and all have turned out excellent and maintained the honey flavor. You can also add at flameout if you're worried about wild yeast or other beasts, but this hasn't been an issue for me so far.
As for how much, that I'm not quite so sure on and it depends on your recipe. You need enough to balance the rest of the flavors of the brew, but not so much that it overpowers everything. The HBT wiki says 10% max in batch, which I think sounds about right. I would recommend starting with less the first time around and then working your way up to see what seems right, but if you're really looking for the maple syrup flavor start with the 10% to see how that works, and then add or subtract as necessary the next time around.
On my chocolate maple porter (5 gallon) I make, I put 3.75 cups of maple syrup (natural organic) in the boil at 15 min, and then I bottle with 1 cup of it. It comes out fantastic.