Did anyone figure out exactly why this happens and anyone can confirm if it got worse for you too? Sometimes it takes like 5+ attempts toggling circuits until it logs in.
There's a time limit, as jackg already posted. As for the "why": some say Google is using the many fast connections made by recaptcha to track users through other means (but I don't know the details of how that works).
As a work-around: if you allow your Tor browser to store cookies, you don't have to use captcha each time. It's bad for privacy, but if you only use that specific Tor browser for this forum, I'd say it's acceptable.
Afaik, only your registration IP and your post IP are registered so you can login from your regular internet connection and then switch your browser to using tor beyond that point (using the 127.0.1.1:9105 or whatever the port is that you need)...
This defeats the purpose of using Tor. You should always assume all activities are registered.
I would like to know how to use Tor.
It's probably much easier than you think: download
Tor Browser, and run it.