Anecdotally, when the benefit is applied, you get the status for the remainder of the current calendar year, the subsequent calendar year and the first three months of the year after that.
You only need to wait until April if you are already Diamond and are going to drop to a lower level at the end of March.
There is no point in applying the benefit if you are already Diamond.
So it is really more the case of waiting until your status drops before applying.
If you do not hold Diamond status then apply the benefit from January for up to 27 months of status.