Just to confirm, 2 days for a GTX 650 is about right. For comparison, my OEM GTX 645 cards (which are faster than a GTX 650--GTX 650 has 384 shaders vs. GTX 645 with 576 shaders) runds the GFN short in about 26 to 27 hours. Thus, depending on your clocks, a GTX 650 should be in the 40 to 50 hour range.
Also, to follow-up on Mike's note regarding your errors, the overclocking might be the culprit, but a memory overclock is probably not the issue here from my experience on very similar cards (e.g., GT 640 cards). Rather, the overclock and/or case environment might be resulting in a lot of heat issues, especially if you are using a stock GPU cooler as many of the coolers on these entry level cards are not very efficient.
One other common issue that arises with these lower end cards is that they don't handle other loads very well while crunching. Unlike high end gaming cards, my experience with entry level cards is that they produce more frequent errors if the computer is being used for other graphics work even if it is not very intensive (e.g., a simple game like solitaire, etc.).