If a work or result is promised to be completed "in a few weeks", is there a legal definition of how long that can take the most?
My gut instincts would say less then two months, but that is just my personal notion.
"in a few weeks", is there a legal definition of how long that can take the most?
No. Hence the importance of being more precise in the contract (here, by stating a deadline that does not lend itself to highly subjective interpretations).
“A few weeks” is, in the absence of a particular definition, what a reasonable person in the circumstances would understand by the term.
That’s the legal definition.
You will note that it is not a specific time. The whole point is that it is flexible to the circumstances. If you wanted a specific time you were free to agree to a specific time.
Without knowing your circumstances 1-2 weeks is less than “a few weeks”, 26+ weeks is definitely more, 16+ is probably more so it’s somewhere in the 3-15 week range.