My engagement letters stipulate I can extend their returns at my own discretion and that failure to respond to a request within 2 weeks by the emails we log at time of the engagement letter or if I perceive that have lied to me allow me to withdraw without penalty.  Most returns are paid at the same time the client gets the returns (with EFILE/COPY watermarks, I do use them and I know many professionals do not like them.) I never pursue deadbeats legally. I have long since stopped worrying about the time lost on a partially-completed return. I lose < 1 return / year after instituting these policies ten years ago.