That's kind of a hard questions to answer. I would not worry too much about when you add it to the boil as that is how you make maple syrup! That being said, if what you want is a recognizable maple syrup character, you may need to go with the extract that is used to make fake maple syrup. The other problem here is that the cost of real maple syrup is quite high and 10 to 12 ozs, which is what I used in the past, costs about $15 or more. If you can get a dark lower grade, I think it will work better for a beer just not on pancakes! If you go with the real McCoy, I'd recommend adding it at the end of primary to get as much of the character into the beer and keep it out of the scrubbing action of CO2. This is a bit hard to do but of course it would be worth the effort!
My Indian name is "Running With Beer"