UPDATED WITH OCT 2012 SALES NUMBERS.
I've seen it mentioned how the Volt is doing versus the Prius during the same period, but I never saw the numbers. So I've compiled them. The data is in the attached spreadsheet, along with the links to the source data.
These are USA sales specific. I show the Prius started sales in July 2000 and the Volt in Dec 2010. I call this "First Month". The first 12 months of sales are "First Year". Then we have months 14 through 23 (October 2012 Volt numbers included).
SPOILER ALERT: The Volt now has monthly sales exceeding the Prius at the same period in its sales history.
If Volt sales continue even at the same rate, the Volt will overtake the Prius in total unit sales over the same timeframe within about 3-4 months. Also interesting is that the Volt actually surpassed the Prius' sales rate back in March on a like-for-like basis. You can see how it's now on an accelerated path well above what the Prius did. July appears to have been the breakout month where things really started to take off.
Spreadsheet is here