My experience has been that the $ doubles every time I finish a survey. I don't have to wait until I reach $25 for it to double as peggster posted above.

This is wonderful, by the way, as usually I would stop answering surveys during the period of time that I will lose the $. As a result I was up to $100 quickly and redeemed it for a $50 Target gift card (which you (well some) can get quarterly), which left .02 in my account. My next Hilton redemption is in August so I'd better answer those surveys I just got.