Most places I've been involved with for underground power or HV radar (e.g 3 mW, 11kV 60 Hz; or a few hundred kW at 40 kVdc) only do a PD test initially, then run the system till there is a failure. Overhead lines mostly have RF detection from the ground on a routine (annual) basis mostly to detect contaminants on the insulators.
There is a French company that has a charge/relaxation test that can predict remaining lifetime in XLPE cables. I'll post a link if I can find any open source data on-line, details in my files from a visit to the folks doing that work at Marseille a few years ago may not have been made public yet.