The PCV valve is Always open.
The flow is more restricted at higher vacuum.
As vacuum drops, more air can flow through the valve.
So...
On a stock v-8 idling at 600 rpm, vacuum is around 17"Hg and say 18 or 20 cfm are going into the engine.
The PCV valve is letting in about 1.5 cfm of that, and it mixes in under the carb.
If the same engine is cruising down the road at 2700 rpm, 17" Hg, something like 30 or 40 cfm may be going to the engine.
The PCV valve is letting in about 1.5 cfm of that, and it mixes in under the carb.
But as the throttle opens for a hill or part throttle acceleration, the vacuum drops and the PCV will allow more air through - hopefully enough to keep the crankcase as a vacuum or at least not going positive. Approaching wide open throttle, that will be something like 4 cfm.
PCV Flow Rates