One part of me wants to say "You are getting plenty of money from 600 properties, and you have not qualified any reason for moving to a monthly test other than you would have difficulty performing what could be considered to be your duty. If I built a 3 storey house next to yours, why should I have to turn up to mine every week to test the alarm when it is easier for me to stay in watching Jerry Springer? If yours are safe with a monthly test, then so are mine. It is almost like me going out and buying 100 cars, then saying that I have too many cars to MOT them ALL." (Half joking, but there is a point in there somewhere)
Then another part of me thinks "How about asking the residents? A small rent reduction for someone flicking a switch once a week might be a fair trade."
From BS5839-6
All Grades of system need to be tested periodically to ensure that there has not been any major failure. This
does not require any specialist knowledge, and can normally be carried out, quite easily, by the occupier of
the dwelling, who will, however, need simple instructions in how to do so.