Probability that, given a flopped set, you will improve to a full house or better after the river: 33%.
This is an very interesting statistic that I did not know. I guess it hasn't come up enough for me to wonder about. But that guy with the 4-flush has 9 outs. And you'll have 7 outs on the turn card, and 10 outs on the river to win EVEN IF they hit the flush.

Thinking a different way: Even if one card of your set matches the suit of their draw, they only have 8 outs instead of 9. At the turn this is reduced to 7.

It's like a really close sports match. You flop the set and a huge lead. They turn the flush and jump way out ahead. But you still have 10 outs(21%) to win the game on the river.

Ideally, the same card that completes their nut-flush makes you the full house. :P :P :P