Can try at various points. Simplest/smallest amount would be to prime with it if you bottle.
It tends to ferment way out, so effectively it's like adding sugar. Rather fun to play with all by itself, but that's more meady than beery, and perhaps best done during sugar season, when you could probably snarf 5 gallons of hot unfinished syrup at about the right SG, rather than have it boiled all the way to syrup and then diluting it back down - assuming your source is a producer.
On the very low end, you can (during the season) use sap as water in the brewing process. Adds a tiny bit of sugar, and a note of flavor, if it doesn't get utterly stripped in the primary. With that stripping (foam fractionating, if you like) in mind, if you want the maple flavor/scent to stand the best chance for a bulk addition, brew a low OG beer and either add to the primary after a week or rack and add to secondary. Of course, this makes the gravity measurements annoying - but primary can really rip favors (and colors) right out of things - I made a cranberry mead once that started a beautiful pink, and ended up with no pink, and no cranberry. Still, if your source is cheap enough that you won't mind possibly just converting syrup to alcohol, tossing some in the wort will certainly work, so long as you allow for the gravity increase.
The few commercial "maple" beers I've tried need to be labeled maple, or you'd never know it....any trace of maple flavor remaining is overwhelmed.