Here's a video I just made that has a short bit showing the case drain flow from the fan motor. This is a motor in good condition and the line only carries the motor case drain flow. As long as your line only has the case drain flow from one motor, it should not exceed the flow in the video by much.

 However, my Flexicoil VR cart has the return from the meter motors combined into the case drain flow line and run full flow from a 1/2" line.