Which brings us to the next lesson (aimed not only at our new friend in Western Visayas, but everyone else who aspires to progress beyond the mere "fire fighting" model of tech support):-
The absolute need to maintain good records.
At the very least:- records of work carried out (job reports).
Then, you can quickly, and with some accuracy, estimate how much time is spent on CM (and, hopefully, PM as well) simply by analyzing the previous years (ten years, whatever) job reports.
You don't even have
to use a computer. A simple ruled log-book, if properly drawn up and diligently maintained for all work carried out, will suffice.