You can still go tens of thousands of miles on a ripped boot. I know of a certain Subaru wagon that has 165,000 miles on it, and the boot ripped at 30k. Sure it makes a little noise, but it will still get you there.

All the boot does is keep the grease in, and moisture/junk out. The length of time it takes for it to fail depends on how much junk you can cram in there and how wet you get it.

If you ever run over a plastic bag, make sure you see it exit the rear of your car. I ran over one in my civic, half the bag snagged a bolt head, and the other half wrapped around my axle. Almost instantly it tightened around the boot and sawed through it. I know that's what cut the boot because I was the one that picked the Food Lion bag out of the joint. Brand new axle, too. It's not unusual for a boot to leak, but a punctured boot is usually what eventually causes the joint failure.