I'm building a kegerator for my husband as a welcome home from deployment present. I'm going to assemble it in about a month and he's back in about two months. I want to connect everything and tap the beer to make sure there aren't any leaks or problems, but if it sits for about a month, is that going to affect the beer?
If you're using CO2 to force dispense the keg, as in a kegerator, then tapping it shouldn't really affect the life of the beer (assuming there are no leaks or whatever). The beer in the keg remains covered by a layer of pressurized CO2, which will prevent oxygen or an infection from getting in.
However, a month is a long time for beer to sit in the lines, so if you do hook it up to test it, I would drain and clean the beer lines with an appropriate cleaner after verifying that everything is hooked up correctly. Or you could just wait until a week or so to go, which should give you enough time to work out any kinks.
How often and thoroughly you clean the beer lines is sort of a personal preference, but if the beer has been sitting for more than a week or two I like to flush it with water at least. Some people are willing to wait a lot longer than that...
(As a side note, if you tap a keg using air pressure to dispense, e.g. with those hand pump dispensers, the beer will stale much more quickly since oxygen is getting in -- those are really for drinking a whole keg at once for a party or something.)
Followup regarding beer shelf life:
The original question was about the effect of tapping a keg, but there is also the implied issue of the shelf life of beer in general. If the beer is stored properly it should be good for at least 3 months, and generally 6 months should be no problem for most beers. Exposure to light and heat can shorten the life of the beer, but neither of those should be a problem in a kegerator. If you get a commercial keg there may not be an easy way to determine how long ago it was brewed. Beers with higher alcohol content will last longer—I've heard a rule of thumb that once you get above 7% ABV the beer will last indefinitely if properly stored (although the flavors may change over time, for better or for worse).