How long should the first conversation be?

As long as it takes to learn their goals and gauge real interest, and not a minute longer. That might be five minutes or twenty five. The mistake is trying to download everything you know into one call. Information overload does not close people. It overwhelms them. Learn what they want, share enough to keep curiosity alive, and set up the clear next step: another call, a video, a presentation. A great first call ends with them wanting more, not needing an aspirin.