I so far have gone three months and the keg stayed pretty fresh. Usually I would go through a keg every month and a half but since the wife became pregnant, my beer consumption is down. Beer technically never spoils, warmer temps greater than 38 degrees can encourage fermentation which can change the flavor. That is all. I use CO2 as my driving gas. Some prefer Nitrogen.
The cost savings is huge. Each barrel holds 165 beers 12 oz beers (6.875 cases). I purchase each keg for $49 (Miller Lite). The equivalent in cases costs me around $14.00 each. Each keg will save you around $47 in beer costs. The CO2 runs me $13 each time I fill the 5lb tank (each 5lb tank lasts about 7 kegs or so). Electric costs wash as you have to keep both cold anyway. So after 11 kegs, the cooler pays itself off and then you are drinking for around .29 cents a beer. Not too bad. Plus it is a great conversation piece, and your circle of friends will grow. The other benefit is less garbage from the lack of cans and bottles to dispose of. Just think, you will be doing something for the environment!!!
Here is a great source of info: http://beveragefactory.com/faqs/draftbeer.shtml