I found very little material on how high one needs to be to do this. The best I found was that one needs to be 1000ft above the peaks, for every 10kt of wind flowing across them, to be sure any downdraught is no bigger than a few hundred fpm. It has proved to be a good rule.
In a mountain wave, vertical currents can exceed several thousand feet per minute, and can extend twenty or thirty thousand feet above the mountain, as well as several hundred miles down range. Typical magnitudes are less, in the order of 1,000 to 1,500 fpm, but they can be higher.
When approaching a ridgeline close to the ridge altitude in a light airplane, the 10 knot/1000 ft. rule is a good one.
I've spent a great deal of time approaching ridges from about 15 feet above the ridge, in strong winds...knowing where to go in the even that downdrafts and rotors exceed aircraft capabilities is far more important than having sufficient altitude above the ridge. Having that altitude is more about giving you options for turning and exiting, than it is for being in an area where updrafts and downdrafts won't affect you. You'll have to fly far higher than that to escape the effects of a mountain wave.