I have the codex x20 plan also. Set it to 5.6 sol ultra, gave it one task, in less than 10 mins I got the 10% warning. I did have it do a few things earlier, just some anaylsis and then create a pdf from it. I thought maybe that took a little longer than I though.

Used a reset, it went for about 11 minutes and then, just out of usage popped up, no warning, there were still 4 agents running, the nice thing is it did let them finish, each one went for about 10 minutes.

I also have a claude max plan, I have been using Fable 5 on ultra, I never hit the session limit, and get 3 or 4 full day's looping on ultra.

I don't know how it handles the subagents, but claude does it much more efficiently, Codex does seem much faster, so maybe it's just a relativity thing.