A therapeutic dose implies one that is sufficient to have a beneficial effect. As such, I expect there is no therapeutic dose whatsoever.
However, what can be said is that...IF dose levels are closely correlated to blood levels (and if memory serves then AVXL claims that they are) THEN we can expect that the vast majority of patients will not be able/willing to tolerate the TARGET doses as determined by AVXL's highly dubious concentration-effect claims.