It depends on how much blowby you get past your piston rings. So it's not an exact point, but it's at higher rpm & more-open throttle. Whenever the amount of blowby exceeds the amount sucked out thru the PCV, then the flow reverses in that vent line.

You could put another catch-can in that vent line. If you use a check-valve you risk pressurizing your crankcase - not good.