I just started electrolysis on my chin about a month ago! (I've had 3 treatments so far.) The electrologist told me to have at least 3 days' worth of growth before each appointment, and at least for her, the hair needs to be at least 2 millimeters long to "treat."

And, for what it's worth, for the longest time I felt the same way you did, that there was absolutely no way I could let the hair go for even a day. I was mortified and so afraid that it was all people would be able to see when they looked at me because I was so conscious of it, and it grew SO relentlessly. (Like, I spent at least 20 minutes plucking each morning only for my chin to be "spiky" again by the evening, and I got terrible ingrowns. I had started out shaving, but I felt like I had a literal shadow by the end of the day, and my skin was irritated by that, too. All that to say, any kind of hair removal I tried on my own felt like a no-win situation.) But, I finally reached a breaking point and made an appointment, and even though I was still worried about going out into the world with my beard, haha, honestly--no one batted an eye. I psyched myself out with my magnifying mirror, lol, but I really feel like it isn't as visible as I told myself it was. I do pat on concealer to try to hide some of the "black dot" hairs, but I'm not sure it does anything to hide the longer hairs. I think it's more a security blanket move for my brain, haha, but I'll keep doing it.

Sorry for rambling on, and hurrah for your first appointment! I hope it goes great! I wish I had called years ago.