Answer:
It depends entirely on your type of monitor and power supply, but you can figure on the average of 100 watts per amp (that's a generalization of course). If you have for instance, a 350 watt power supply, that means the power supply can put out 350 watts of power for the computer to use, but that's the low voltage components of the power supply.
As an example, I have a CRT type of monitor that uses 2.5 amps but on my power supply, there isn't a rating tag, so I'd approximate it at 1 to 2 amps.
People tend to confuse output watts as the draw of power from the AC voltage, but there are other variables that come into play.
Yes, those variables are power supply efficiency, PFC but mostly follow ohms law.