A common problem with design iteration (evolution) is the absence of a defined endpoint – i.e. each iteration refines the design, but it also reveals additional opportunities for refinement, resulting in a design process that never ends. To avoid this, establish clear criteria defining the degree to which design requirements must be satisfied for the design to be considered complete.
-100 Principles of Design.
Getting final client approval can be really difficult. New designers tend to think it is just their work not being good enough. Well, that may be true. But rarely is work perfect. However work does need to come to a stage of completion that the client can agree on – doing this near the end of the project can prove costly and tiresome.
Keep your work to a minimum and keep your clients happy by keeping your limitations realistic.