Power output from USB 3.0 is up to 900 mA (1 unit load = 150 mA for USB 3.0, and a device can draw up to 6 unit loads from a USB 3.0 port; 150 mA unit load x 6 = 900 mA).
It depends on a couple of things, like BIOS (ACPI) mods or running ASUS USB Charger that allows you to pull more juice from the ports.
I've tested and noticed for example that the two ports from top/left side of my laptop are not that "strong", one of them is also dropping the voltage from 5V to 4.5V-4.7V pretty fast under load. So I'm not using both ports from that side at same time if I have some hungry devices.
