There is a saying “If you have 10 hours to cut a tree, spend 8 hours sharpening the axe”.
Similarly, launch when you know you have all the competencies and the resources required to run the business. I.e. when you have worked out your business plan, evaluated the business case, spoken to customers and are convinced that the value proposition makes sense to them, when you have tested the product, when you have understood the dynamics of marketing & sales, when you have evaluated the cost of acquiring customers, when you have identified – and some what validated – all the assumptions that you have used in your business plan…. that is the time when you are ready to launch. And of course, you need to ensure that you have the required capital to sustain the business till you either (a) hope to become self-sustaining of (b) when you hope to raise external capital – whether as a loan from banks/family/friends or as risk capital from angel investments/ VC/family & friends.
HOWEVER… despite all this, and even after you are ready, you will have to evaluate what is the best time to launch. E.g. if you are selling something to schools which they will use in their classrooms, launching in the middle of a school term may not be prudent.