Service request level retention by default is configured for 35 days and the following is documented for Timeseries metrics.
A web service that is configured as 'Key Request' as well as 'Multidimensional analysis views'.
We do not have metrics data of that web service for more than 35 days which tells us that the Service request level retention is applied to this request.
When and how the Timeseries will be applied to Key Request and Multidimensional analysis views?
Multidimensional data has only service level metrics available from transactions. You can create Custom Metrics (or soon will be able to) based on those data and than you will have better data retention.