I believe 2800mA's to get 900 lumens. There is some comments as well in DX's site:
DealExtreme: $15.37 SSC P7 C-Bin LED Emitter with 21mm Heat Sink Base (3.6V~3.7V)
But I really don't think you will get that much current draw from a single 18650 unless there is a driver that can do that. I have tried three different drivers for the P7's that I have. (three different lights. Two mods, and the SpiderFire one) And I can only get that much current if I use 2 X CR123 with the Kaidomain driver. (which is 2 X AMC7135's)
P.S. I'm sure that the P7 lights on DX that use 2 X 18650's will give full power. (I would imagine)