warrantys are a little hard to do if you dont price the job to do it "properly" haha, this means removing all peeling paint and so on. if you do so then warranty against paint peeling blistering or flaking can be there but only if your using premium products . it comes down to the old u get what u pay for theory.
we do a 5 year workmanship warranty which only covers our workmanship as an applicator . so any cracking or paint blistering due to bad formulation etc is not our problem... i believe its fair to offer something but if your not into it then thats all good too. i personally only offer it for expensive quotes or new work.