If Applebloom is anything to go by, absolutely none of the cutie pox marks gave anything particularly useful. Even if it did, the other marks would likely make it useless to try and make it effective, since the victim would likely have no way to record the knowlege they were temporarily given.
I just rewatched the pox part and I think this mark was definitely useful:
http://imageshack.us/photo/my-images/830/applebloom.jpg/While having only 3 cutie-pox-marks she was still capable of performing all 3 of them simultaneously.
How long did this last?
1 >> The ponies are playing before the start(?) of school classes . So this likely happened in the morning. (1CM)
2 >> She is at home. We can clearly see how it nighttime when looking out the window (2CM)
3 >> Then we see Applejack incapable of sleep because of the step CM. They visit Twilight to ask for help. (3CM)
4 >> Suddenly it is daytime and we see AJ and TS running on the street.
Judging by this at least 1 day passed where she was fully capable of performing 3 CMs. It could be longer, considering she caught the pox the day before(?) while visiting Zecora.
Assuming that not everypony's immune system works equally, there are probably ponies having either more or less time enjoying their amazing new capabilities.
And even after getting one after another in rapid succession, she was still capable of performing each one for at least a few minutes.
But you are of course correct in that one reference case is not enough to prove this as true.