[feat] max/duration option to use a function

I'm willing to writing a feature where you can provide either an integer or a function for max and duration.
It could work like the "id" field where ctx is available.

max: Integer|function
duration: Integer|function

Would you be open to this feature?

Start by reviewing how the existing id option makes ctx available; the issue does not name a file, entry point, or test. Clarify the behavior for function-valued max and duration, including when each function runs and what constitutes done, then add tests covering integer and function inputs.

Written by the indexing model from the issue text.