This is a good question and I am curious to hear other shops answers.
Out here in California, there is no lifetime expectation for a vehicle wrap. Usually our customer has an expectation of 5 years (if they've done a little research online) or no expectation at all.
The warranty you offer should be the same or less than the brand of vinyl you offer. That way, you're not responsible should there be a defect in the materials. If there is any lifting of the graphics, that's usually installation error and the responsibility of the shop to fix. Excessive shrinkage, streaking in the media/laminate, premature fading or spotting are manufacturer warranty problems.
We are one of the few shops that don't use 3M. We've wrapped over 70 vehicles the last 3 years and the only failure we've had is on the perforated window vinyl. Bottom line is to CYA.
-Bud