Well, it took me about 9.75 gallons since I brew all grain. Here's the recipe I went with (BeerSmith should be able to convert, though I can't attest to how well):
6.5lbs White Wheat Malt
5.5lbs 2-Row
1lb Carapils
1lb Crystal 120L
Mashed at about 154
1oz Wilamette @60
.65oz Wilamette @20
Fermenting with Wyeast British Ale II (came up with this after talking over it with the guy at the LHBS)
That works out to just over 14% in crystal malts, mainly because my buddy who tried this last year found that the sugars in the sap tended to dry out his beer, so I'm using more crystal than I normally would to try to counter that effect - hopefully not so much that it'll compete with the maple though.
EDIT: And for reference, the brew day went quite well, despite a significantly increased cleaning load... I filled up a pair of better bottles with sap, so both of those had to be rinsed with hot water a couple times and they're currently soaking with Oxyclean. My HLT, which rarely ever gets cleaned since it only really ever sees water in it, got a really got scrub down, and all my hoses, the detachable valve for my pump-in-a-box, and anything that will fit, are all soaking in another bucket of oxy now too, in addition to all the normal stuff that needs scrubbing and soaking and cycling of oxy and rinse water, etc.
But the brew itself? Interestingly, I thought I was walking away from the maple farm with about 10.5 gallons, but I was measuring on an incline, and actually wound up with closer to 9.75 into the HLT. This was a little too close, so I added .5 gallons of water just to be safe (I hated to dilute, but BeerSmith told me I needed 9.73 total). The sap itself was mildly sweet, but not noticeably maple. Anyway, after taking my first runnings, I found that the grain didn't seem to absorb as much liquid as BS projected, so I figured what the heck, and sparged with every last drop I could, and boiled 90 minutes. Turns out I didn't need that extra water after all. The resulting wort did indeed taste of maple, and I'm hoping that sticks around post-fermentation.