As long as the valve is clean and not coming apart, it is not necessary to change it at all. I still have my original at 190,000 miles. I believe in agressive maintenance and if there was any doubt about it's condition it would be replaced. I clean it about every 60,000 miles, usually at the same time I am cleaning the throttle body. (I clean the throttle body much more often though)
The main reason for changing the valves years ago was from them getting sludged up due to the crappy oil of years gone by. Even then they could often be cleaned. It was my practice as a tune up tech to include the PCV valve in the cleaner dip when overhauling a carberator.
One of the main reasons for not replacing it, is that it is easy to end up with one that is not intended for your exact engine and it will either flow too little or too much air and at least slightly mess up proper fuel control.
So I would rather have a clean and properly operating original that a questionable replacement.
That's my $.02.
