Another rule of thumb is the total wind versus the clean stalling speed.
Dick Collins (or someone similar) wrote that when the wind gets above 50% of the clean stalling speed, you ought to start thinking about it.
In a PA28, that would be circa 27kts.
Personally, I wouldn't fly in those conditions in a PA28, as much for comfort as any safety considerations, it would probably be quite turbulent and uncomfortable for the pax.