Maximum power rating for Arduino UNO revision 2 (SMD)
The Arduno UNO SMD R2 uses an MCP33269 not an LM1117.  What you have is probably some cheap Chinese clone of the Arduino, and in that case the regulator will be the cheapest Chinese copy of the LM1117 available.  It's impossible to tell what the specs of that specific chip are.  Suffice it to say, though, that the specs of the chip are pretty much irrelevant since they rely on good heatsinking, which you most certainly don't have on a little Arduino board.

500mA, though, is a lot to push through a linear regulator like that, and when running from batteries the excess power loss imposed by the regulator will be a major factor in longevity.

You are better off investing in a proper switching regulator (such as a UBEC from a model aircraft supplier like Hobbyking).