I tried offering a lifetime warranty, but I would have charged more for the warranty maybe 30 percent and only on certain parts. I had it in my ad for 1 year, and guess how many people asked about it?..........Class?? ...............NONE. It said "Ask about our lifetime warranty on parts". Nobody asked. The theory was that if you play the odds, you probably never would have to warranty said part. One hook I would have told them if anyone had bothered to ask - they had to have the original paperwork (invoice) and "WARRANTY" certificate you would give them. I made them up and numbered them. Looked like a Diploma or something like that. Again, the odds are that in 4 or 5 years, who would keep or be able to find the original paperwork. But nobody even asked. Or, they would forget about it.
So, I never mention the warranty unless they ask. If they do, I tell them warranty is 90 days, but service call and labor is 30 days. Now, if they had a problem within 90 days, I would tell them to be nice I won't charge another service call or the labor. After all, it wouldn't be their fault part failed. I would just warranty it, get another part for free, and be done with it. Want to keep customer happy.
I have little stickers made and keep them in my invoice book, BUT only put them on the invoice when someone asks about warranty. Now, I can count on one hand the amount of warrantys I have given out in the last 5 or 6 years. Almost none.
Some of you might want to consider the LIFETIME WARRANTY as a marketing tool to get a leg up on competition in your area. 1 year or 2 year also consideration, but charge a premium of 20 or 30 percent for the higher warranty. I've been considering that, sort of like the extended warranty a new appliance store sells. Again, Consider the ODDS. Good Luck