Ah quite a good question….
I believe it depends on the steeping method.
Highly oxidized oolongs can be steeped many times if the proper amount of leaves is used with the right amount of water. For darker oolongs, gongfu steeping is the way to go! I have easily got 12 good steeps from a pearled, dark oolong.
Formosa is different though because the amount of times one is able to steep a tea is dependent on the surface area of the leaf.
BaiHao (formosa) oolongs are not highly compressed or rolled or pearled. They have a more wiry appearance to them, so they do not have as much surface area to be exposed during the steeping.
Does that make sense?

William,
these impressions are partially correct, but don’t hold for all oolongs. The two styles for oolongs are balled (or rolled, like Tie Guan Yin) and strip-shaped (like dancongs or Wuyi Yancha). Both types are good for many, many infusions in general. A decent Da Hong Pao can give you anywhere between 8 and 15 infusions, while a dancong usually doesn’t go quite as far. Same with balled oolongs. Da Hong Pao has a fairly high oxidation level, depending on processing method.
“Choice Formosa oolong” usually describes some form or Bai Hao oolong from Taiwan. These Oriental Beauty oolongs are much more akin to Darjeeling “black” teas than to other oolongs. My experience with Bai Hao oolongs is that the most satisfying flavours come out in the first 2-3 infusions.
If you want to prepare these teas with mutiple infusions (i.e. more than 3), use MUCH more leaf than you’d usually do and do VERY SHORT infusions (i.e. a few seconds). That should keep you going for a while.
Of course, quality is also a major factor in this issue…

I agree with yaya, though I do not have as much experience with Bai Hao. With yan cha I use about 6-7 grams in a 60 mL gaiwan, start off the first infusion at about 5-10 seconds, then gradually increase steep time. I can easily get more than 12 infusions out of it this way. Quality plays a big role in this of course, but generally when i brew with this type of kung fu the above is standard.

As far as formosa oolongs go, I generally find on average that they do not go for as many infusions. This may have to do with processing but I’m not quite sure. I would say that there exists a correlation between oxidation and number of steepings when using the same amount of kung fu. But as yaya said above, quality is of course a major factor as well.

Hope this helps in your tea journeys and in honing your kung fu with more heavily oxidized oolongs.