On a $20/mo plan doing any sort of agentic coding you'll hit the 5hr window limits in less than 20 minutes.
It really depends. When building a lot of new features it happens quite fast. With some attention to context length I was often able to go for over an hour on the 20$ claude plan.
If you're doing mostly smaller changes, you can go all day with the 20$ Claude plan without hitting the limits. Especially if you need to thoroughly review the AI changes for correctness, instead of relying on automated tests.
With Codex it only happened to me once in my 4.5hr session here: https://simonwillison.net/2025/Dec/15/porting-justhtml/
Claude Code is a whole lot less generous though.