Usability evalation can take place at any point in a site’s lifecycle. One common time to evaluate accessibility is on a live product when considering whether a redesign is necessary, or have already decided to do a redesign. In that case, the evaluation can serve as a benchmark to see whether the redesign is successful. Evaluating competitors can also be helpful for the same reasons. Another common time is while a site is being designed, to ensure that designs will lead to a successful product before development, when changes become more costly.
A variety of methods exist for evaluating usability. The gold standard is to test a production site with real, in-person users. Such tests can be expensive, but the data they produce is invaluable. Less costly methods include testing visual designs or conducting a heuristic review. Even cost-constrained projects should include some testing to validate they are on the right track.