Before I joined Akai and was looking for roles, a startup's founders reached out about one. We had three calls, and on every one I walked them through how I'd approach their go-to-market, all of it for free.
Then it took them five days to decide whether we were even going ahead. The role went from full-time to freelance to a no, and every time I checked in they were still talking it through.
Instead of an answer, they asked for another session because it would be "easier to do live." That would have been a fourth free brainstorm, so I said no and offered email.
They wrote back "Haha this isn't gonna work. Thanks for your time." I said likewise.

That wasn't the only time. Another company I was working with signed a three-month contract with me.
Before that I'd done three 45-minute strategy calls with them and built real work for them. After we signed, I delivered multiple iterations of the positioning work.
The whole way through, things kept getting held up, my experience got questioned and the work kept getting edited. Then I got a message saying they were parking the project for now because of their "capacity."

They did say I'd be paid for the work to date. That turned out to mean one month, not the contract I'd signed.
Some a**holes would have pointed at the contract, asked for the full amount and fought it. I dropped it, blocked them and told my friends not to work with them.
These are just two instances. In my 11 years of work, if I got paid a dollar every time this happened, I'd own a penthouse.
So my rule now is two calls, max, and I try to start async, a few questions in a thread or over email, so I can see fast what the other side is like.
If it's not working after that, I drop them and move on. There's no hard line, it's mostly the vibe they give off.
When I'm hungry and the company is really good, I'll do more. I did for both companies in this story and for one more, and sometimes it doesn't pay off.
My first product went that way too. The freelance devs and interns I hired for it didn't give a rat's a*s, and it cost me a large part of my savings.
These are the two lines I used on the first thread, if you want to steal them:
I'm happy to keep answering questions over email. We can do a proper brainstorming session after we get started next month.
If you're hiring and it's a no, tell me on day one and I'll thank you for it. I won't keep working for free while you decide.