Well, it's fermented, so its shelf life is like that of yogurt (i.e. long!) I had my finished kefir in the fridge for 10 days while I was away, and it was just fine when I got home. If you look at Dom's site (listed in the original post) he says he's kept a batch for a year in his fridge and it was still drinkable.

Keep in mind, kefir was originally made in a leather sack, hanging in the sun all day and inside all night. They just added milk each day to replace what they drank, so you can imagine how old some of it got. It's very, very forgiving stuff.

One other detail: if you want to go on vacation or something, just put your fermenting jar in the fridge. The cold slows the process down so if it normally takes a day to convert, say, a quart of milk to kefir on the counter, it can take a week or more in the fridge. I left mine for 10 days and it was just fine. When I got home, I strained the batch into my gallon jug, added fresh milk, set it back on the counter, and within a day it was back to its normal quart-a-day pace.