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 people treated with osimertinib, resistance usually develops within approximately 10 months.
https://en.wikipedia.org/wiki/Osimertinib