According to Wikipedia, it depends on a specific cancer cell mutation, T790M, which could also be a forced mutation from using other EGFR tyrosine kinase inhibitors. This alludes how tricky, and how fast, cancer cells mutate and adapt.
>> In people treated with osimertinib, resistance usually develops within approximately 10 months.
In my MIL's case it was L858R. T790M also present, effectiveness of drug dramatically increased with presence of both. Osimertinib shrunk the tumor from baseball to golfball, and then her left most lung lobe was surgically removed. She was stage 2 at intake and without metastastis. The drug was not approved for that case in 2021 and she was admitted to an ongoing case study for it.
My sister had the ALK variant of non-small cell lung cancer. The special treatment she got worked until it didn’t, about 16 months. One oncologist said the cancer had likely mutated and the ALK specific therapy was no long effective.
>> In people treated with osimertinib, resistance usually develops within approximately 10 months.
https://en.wikipedia.org/wiki/Osimertinib