Ideally CRO should be an ongoing process of continually trying to refine and improve your website. Every month as a minimum you should aim to analyse the site, come up with some new testing hypotheses and run a new test. If you are planning a new website, use CRO as a way of testing ideas on your existing website to decide whether they should feature in the updated site. If you’ve already invested in a new website, don’t just sit back and expect it to work optimally, closely monitor performance and get testing as soon as you can.
There are many different views on this, because areas of conversion rate optimisation and user experience do overlap. We like to think of them in this way: CRO is the process of refining and improving your existing website through on-going A/B testing and optimisation (it is also typically focussed on your business objectives). UX is the process of planning and researching the best way to make a significant one-off change, often in the course of building a new website (and it is usually more focussed on customer objectives).