My apologies, I completely misread your question. I thought you asked "volts" not "watts." My bad.
From my testing I've found up to 12V you can draw up to 300mA through Vin on a R3, and 500mA on a R1.
Commonly we see people who report "my project works on USB but not with 12V." This is because 500mA is pushing the regulator too hot through Vin.
I estimated the on-board regulator, as laid out, could dissipate around 1W to 1.5W before shutting down, if I remember correctly.