When my paint gets feeling rough (needs claying), and when all the dirt/bugstuff isn't coming off _completely_ after a wash, and those pesly scratches start showing up....then it is time for me to start over. Putting sealant over a dirty surface is not a good thing.
Sometimes I spot clay, or spot-buff a scratch or bird bomb etching, then reapply AIO and SG, but not very often. Here in South Texas, a 3-coat Klasse job, weekly washings, along with monthly spritzes of Sonus, lasts about six months, before it gets redone.