Used to do every quilt. Now just occasionally with no set frequency but I don't quilt full time. Definately if I start getting thread shredding (usually means a burr) or breaking often, if it pops going into the fabric, or if I've just done a quilt or practice with lots of hevy background filler. I also unless it's just been changed when I do my monthly clean the rails. When I quilted full time at least once a week .