How long is it possible for leukemia to go untreated and still be able to be cured?

It depends: Chronic leukemia as it name implies can be a slowly progressive disease and might be present for years until picked up by routine tests or the disease starts picking up in aggressiveness. Acute leukemia on the other hand advances within a matter of weeks so there is little time before significant symptoms appear.

Impossible to say: The only thing that can be said for sure is the earlier a correct disgnosis is made and treatment started, the better the chance of complete remission and/or cure.