“I will have been working here for 20 years by the time I retire “I will have worked here for 20 years by the time I retire" The 2nd while correct, isn't quite natural for my ears. Perhaps I'm biaised and it's a regional/country thing. But for me, I'd say 'would have', rather than 'will have' in this instance. "I would have worked here for 20 years by the time I retire." Then (for me) the 2 sentences would be equally natural and just different ways of expressing the same meaning.

1. “I WOULD have been working here for 20 years by the time I retire Describes a situation that would be in the past by the time the future EVENT arrives. When you retire whenever that is in (x) years months weeks whatever time, your working would have been in the past. Because at/on/upon retirement your working life ends. An easier sentence is: "By the time I retire I would have worked here twenty years" 2. “I will have worked here for 20 years by the time I retire" your work will have finished when you retire, will + have + (x) = the event (x) ends in the future as stated by the time you retire. An easier sentence is "WHEN I RETIRE I will have worked here for twenty years" And for daily understanding nobody would make a distinction. Your two sentences using will only where one of mine used 'would have been' are the same but I suggest you use WOULD have been for sentence 1 and "will have worked here" for sentence 2. in order to comply with many grammar teaching books. Or simplify usually by reversal of the sentence structure to get a more straightforward sentence, that is more positive.

Hmmm... Two responses suggest using the past conditional for a completed future action. It's probably a regionalism. Here's the answer for exams: I would have worked here <<== hypothetical past result (contrary to fact) More information: https://www.ef.com/ca/english-resources/english-grammar/type-3-conditional/ I will have worked here <<== completed future action More information: https://www.ef.com/ca/english-resources/english-grammar/future-perfect/

I am a native speaker from New York and they sound the exact same to me. There is no meaningful difference. Use whichever sounds best for you.