When do you know 'your ready'
Yep, it kind of is a how long is a piece of string question. It depends on the person and the resources available to that person.
I find that people fall into a bunch of categories.
1). They already do it. They're able to do it.
2). They already do it. They don't have a clue on what they're doing.
3). They think they're ready and they have no chance.
4). They think they're ready and they are.
5). They're not sure they can do it and they can.
6). They're not sure they can and cannot.
7). They're sure they're not ready and they are ready.

. They're sure they're not ready and they're right.
You never really know until you're at #1. It's those false positives the cause the most problems, especially those at #2 and #3. Those people who do it/think they can do it and absolutely cannot do it.
Without a network of experienced peers, the only real way you're going to find out if you're ready is to try and see if you succeed.
There are indicators that'll tell the more experienced whether you are ready. If you have a network of experienced peers, they'll be in the best position to tell you and/or help you with what you'll need to make it all work.
I saw a joke the other day. There's only 2 entry level jobs in filmmaking: PA and Director. A useless director can still successfully make a film if they surround themselves with the correct people AND give them the authority to do the required job.