Depending on whether you're looking for an embedded product designer for discovery on research to production, a designer to explore product directions conceptually, or a designer for an end-to-end feature on www or native devices — the scope will easily vary. The more ambiguous the project, I would generally suggest for teams to partner on retainer basis so there's room to ask questions and even pivot where necessary. The more scoped and finite a PRD is defined, the easier it is to work with a PD for end-to-end features.