Well, that's not very long for a digitally cast printed and laminated job, even for California. I would sooner think, you should get 4 to 5 years out of a good print job. I don't think your customer should hafta pay for a new sign. Perhaps, you could reprint and work a deal of some sort with him for maybe 50%.
If you bought 50,000 mile tires and they only lasted 30,000, they'd be pro-rated.