The following study suggests repeat buyers range from 18% to 30% with an average of 19.9% (depending on the deal site). Loads of other interesting data are in this study.

"How Businesses Fare With Daily Deals: A Multi-Site Analysis of Groupon, LivingSocial, OpenTable, Travelzoo, and BuyWithMe Promotions"

Source: http://www.ruf.rice.edu/~dholakia/

However, I would apply a haircut since this is based on merchant perceptions, which may be overly positive (people are likely to think more highly of something they've spent money on).