"when is the right time" I think depends on what you're putting behind a paywall.
Like, the biggest issue with paywalls isn't whether people like them or not, imo, it's one of money vs. reach. If you put out content for free, it has greater reach, because people can access it, enjoy it, word of mouth spread it, stumble across it, etc. If you put content behind a paywall, you get money for it from folks who are interested, but it also has much lower reach -- the people who pay for it are people who already know you and your work and are already invested enough to know they probably want to see this.
So if you're putting your main content behind a paywall so that it can't be seen for free, you're hugely restricting your reach unless you already have a huge audience -- nobody can stumble across your work anymore, or discover that they like your content prior to paying. If 90% of people who come across you never pay for your content, then only that tiny 10% can become invested in you.
If you're putting extra content behind a paywall so that it can't be seen for free, that's much less risky -- people can discover your main content, enjoy it, and become curious about the extra content. But if 90% of people never pay for the extra content, you're still okay, because those people can still become invested, enjoying and spreading the word about your work.
I don't think there are any hard-and-fast numbers of "once you have X subscribers you can make patreon exclusives" -- part of it is gonna be just going into it naturally and judging your own userbase. You start a patreon and see what people are interested in, you tweak it as you realise what's enjoyable for folks and what tiers go unpledged, you see what kind of exclusive content your patrons might like, etc.
And part of it can be observation -- while no one can give you a magic number, you CAN go look at other series that have patreons -- see what kind of tiers they're offering, see how many pledges they have. Look at different levels of popularity on here -- "okay, here are some comics with 1000 subs -- how are those patreons doing, what kind of tiers do they include, and which tiers are doing the best?" I've done this before to get a general idea of what percent of subs tend to pledge (I don't remember my numbers anymore but it's a small percentage). Looking around at what's working for others can help you determine what's most plausible for you!
