Considering that it's supposed to follow HDMI specs, it should provide at least 55mA according to the HDMI specs.
http://www.hdmi.org/learningcenter/kb.aspx?c=13#42
Other sources say it's 50mA, but I'd rather believe the HDMI website.
A quick test on HDMI pin 18 shows some interesting results:
On the Pi1A, there's a 0.14v difference between GPIO pin 2 (4.98v) and HDMI pin 18 (4.84v) which I think is evidence that HDMI pin 18 is not connected directly to the 5v rail. After a quick check, it's connected to D1?
Update: I've re-done the test on another Pi1A and it's consistent with above.
On the other hand, testing the same pins on the Pi 3 gives no difference (both 4.96v) in voltage whatsoever, giving the impression that it's directly connected to the 5v rail. I also can't follow the PCB trace.
Update: Pi1A+, Pi1B+, and Pi2B are all consistent with the Pi3 result.