How to query Mirroring RTO & RPO from system objects
sp_dbmmonitorresults will generate the data you're after. You could either periodically log it to a table (if you want to gather historical data), or run it ad-hoc, storing the data in a temp table to be able to query to derive your desired metrics. 

You can also use Perfmon counters to track this if you want to integrate it with a monitoring tool that relies on Perfmon.

As an aside, I would avoid using RPO and RTO in this circumstance (despite what Microsoft says). Mirroring/AGs should be treated as high availability, not disaster recovery. A user dropping a table cannot be protected by an availability group or mirror - they will happily replay that mistake on your secondary nodes. 

Using these terms with people not familiar with the technology could be misleading.