From a single cell? Or cells in parallel. As hard as you can.
From a buck driver. I would say 5A. I have fried an XP-G2 at 5,3A after some use, but that might have been the drivers fault..
Djozz have tested them at much higher current. But output does not get that much higher than what you pushing beyond 5A.
Personally I prefer XM-L2 for a thrower with a well sized reflector. The throw difference is not that much when the XM-L2 is driven harder. And the XM-L2 is much easier to push harder. And you get considerably more light. The XM-L2 is way more efficient at "extreme currents", almost twice the light output compared to XP-G2.
In (small) aspheric lights, dedomed XP-G2 is superior to XM-L2. Also, in smaller lights you may not have the surface area to get rid of the heat a hard driven XM-L2 generates. So XP-G2 can be preferable..