Created user stories for a specific software development project and, from my last question, learned to just jump into dev even though there might exist undiscovered user stories.
Still, i wonder... When to know I'm ready to start with development?
This is really a question for the team to discuss amongst themselves and decide on what feels right.
If you are using the Scrum Framework a good guide is that you have a minimum of enough ready user stories to cover the first sprint.
Keep in mind that if you only have one sprints worth of stories that you will likely need to spend a sizeable fraction of the first sprint preparing for the next sprint. So try not to take in too much work so that you have sufficient time for this.
Going forward, the number of ready stories you have will depend on your team's approach and on the nature of your organisation. Good advice would be to just get started and then adapt if you find you are often running out of prepared stories.
The development in itself and the way it is organized, is left to the development team.
So, if your question is when to start the Sprint or the Timebox (the first step of which being a planning or kick-off), I would say it is a conjunction of the 3 following points :
That being said, you should always be ready to start the next sprint when the current sprint ends.