Uber Drivers need to provide an average of 60 rides per week to make $50,000 p.a.

"After looking at the numbers, NerdWallet had a few big takeaways:
Based on their data, drivers can, on average, make more working for Uber than for Lyft and Sidecar in most places. Uber drivers make an average of $15.97 per ride with Uber, as opposed to Lyft ($11.48) and Sidecar ($13.35).
In order to make $50,000 a year, an Uber driver must provide 60.21 rides every week. A Lyft driver would have to provide 83.76 rides, and a Sidecar driver would have to provide 72.03 rides. To put that in perspective, 60.21 rides each week equates to between 20 and 21 hours of driving per week for an Uber driver.
To make $100,000 a year, drivers have to give 120.42, 167.52, and 144.05 rides every week on Uber, Lyft, and Sidecar, respectively."

Source:  Research by NerdWallet, reported by Business Insider, 1st December 2014

Note 1 - As ever, if you want to take a holiday you need to factor that into your other weeks

Note 2 - Based on current US rates