As Salon's Broadsheet inevitably notes, the answer is "Pretty low." From the original article:
When limbo time rolled around, two greased-up dancers in red bikinis and furry Yeti Ugg boots were replaced by the six contestants. "My Humps" by the Black Eyed Peas bumped out of the bass-heavy speakers and the hopefuls began to bend over backward for new breasts. The difficulty was ratcheted up by the fact that the women had to limbo toward the audience. Even though almost everybody fell and confusion reigned, four winners emerged.
One, a tiny young woman named Tracy Boi who accompanied her spirited limbo techniques with gracious kisses blown to the audience, seemed particularly excited. "It's always nice to get free boobies[...]"
Well, we can't disagree with that. Seriously; we've tried really hard. Anyhow, have a look at the video!