I have not faced this problem. I say that there will be a learning curve to understand the product before a new developer can start fixing bugs or adding enhancements. Managers have always understood it.
For contractors too the learning curve is part of the deal. The contractors get paid by the hour when they are climbing the learning curve. Managers seem fine with it.
What is typically considered an acceptable learning cost / duration in your experience? I am not asking because I do not agree that there is a learning curve (we call it assessment), I am asking because I am curious what people expect.
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.
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.
For contractors too the learning curve is part of the deal. The contractors get paid by the hour when they are climbing the learning curve. Managers seem fine with it.