I check regularly but trim rarely. Some sheep, even within a breed, need doing quite often and some never do. I trim the front of the foot - just the flap or point if they're too long - more to remove the pocket that can collect soil and lead to shelly hoof than anything else. I lay the foot shears flat against the sole so the horn is always a blade's-worth proud of the sole, that way I don't trim too hard. It helps that I vaccinated the flock with Footvax, once, in 2007 and haven't seen footrot or scald since