It's hard to find out, since you have to pay for the required standard documents from VESA.
It's possible that there is no fixed value, and it's different for different graphics cards to
implement as they choose. Since (I think) it's required to power the EEPROM in monitors for PnP, I'd say there may be a minimum value of ~10mA ("covering" all versions).
However, looking around I think up to around 100mA will probably be safe. Here are some links that support this:
http://www.vesa.org/vesa-standards/standards-faq/ - See bottom 3 questions.
Discussion of USB power from VGA port specifically this quote:
I found this from http://www.circuitprotection.com/04Databook/C17_video_(133).pdf :
“Devices that comply with the DDC host system standard typically
provide supply voltage on pin #9 of the standard 15-pin VGA connector.
The voltage is 5V ±5% and supplies a minimum of 300mA to a maximum of
but for DVI:
Per the DVI spec R1.0, the “+5V signal is required in a DVI compliant
system… the power pin must be able to supply a miniumum of 55mA and
the monitor may not draw more than 50mA.”
To be on the safeside, don’t do this hack on your DVI port.
Maxim demo board that runs from VGA power (using ~10mA)