There's no "duration". There's "what's the proportion of time you'll spend learning". Unless you have a very repetitive job, it will never go down to 0. It will just move towards it over time.

And the rate of change depends on the person as much as on your internal structure, documentation, scope, etc.

> What is typically considered an acceptable learning cost / duration in your experience?

How long does it take to do a crossword puzzle or play a game of chess? As with everything, it depends.

This depends greatly on both the Seniority of the position and the complexity of the code base in question. Generally jobs I've worked have expected it to be on the order of 1-3 months, although I've found it hasn't always taken that long in practice.

Unless you work on trivial codebases you never stop learning. The code changes faster than you can keep up. It's a constant overhead for everything you do.