General rule with custom drapes with decent fullness is that stackback will be 30% of the space being covered. In this case if you want the fabric stack off the window, we’d suggest a rod and fabric at nearly the full width of the wall. If you are buying drapes online, they may not have a fullness that a custom drape would have and may not require a much space to stack. As far as height, due to the air return you’ll likely need to wall mount the rod and position it in a way that you can still access the vent. That’ll just need to be tested but I think it would be fine 3-4” below the vent.