As soon as your design is complete, it is useful to assemble a prototype and to evaluate it.
Why? It will most probably take several month for developers to implement it but you can find and fix some problems. When design is implemented in prototype, give it target users to use.
- Consider another sample so users have something to compare the prototype with;
- Define evaluation metrics to see how much the proposal is different from the reference;
- Define acceptance criteria;
- Choose participants according to personas;
- Choose user tasks from the core use-cases;
- Document discovered problems.
Design release and a start of the next step in the project (development) or if needed recommendations for improvement and one more design iteration.
While developers go on with implementation, consider several evaluation sessions along the way. Compare measured metrics. Make sure the design/product is still usable after implementation.