i have two chusan palm trees how and when do i know to cut of the leaves

I think this is trachycarpus fortunei - if so it has a hairy trunk. I have one that I bought in a small pot for less than 50p before 1972. It is now a good 5 metres high. I cut off the leaves when they no longer look nice - if they are frayed or brown. I do it at any time of the year but not too many at once. This palm is really hardy and very striking when it gets big.