I have often been asked about the timeline to completely do UX. It can’t really be fixed on a magic number, because it depends on the resources and the level of effort involved. But I can say this, it is an Agile and thus Iterative Process and can stretch weeks to months depending on the level of the engagement.
I can only posit that it depends on how successful you wish your application to be, can depend on the engagement of key critical people at certain points within the timeline of a project. Waiting until the end of a project can have disastrous effects, but inserting them in the right points with the right tools can greatly enhance any project. The timeline I am providing is just a sample and not meant to be set in stone, but it is a common theme I have found when working on a UX project at any company site. So take it for what it’s worth and see if you can apply the concept to improve your project.