How long after being in remission with leukemia are you be considered cancer free?

Difficult question. Part of this depends on the type of leukemia: acute or chronic. The standard thought is 5 years for acute leukemia. However, the risk of relapse is significantly high in the first 2 years, so every year after this reduces the risk of recurrence significantly if you are followed closely by your oncologist.
5 years. I assume by cancer free you mean cured. For acute leukemias we consider a cure 5 years.