For maximum protection and grade change prevention, it is best practice to change milk filters after every milking.
By changing the filter each milking, sediment build-up is kept minimal, thereby preventing potential grading. It is not only best practice, but also more cost effective than cleaning and reusing filters, as it takes less man hours to change out a new filter that it does to clean and dry old ones*. Old filters let sediment build up in the stretched fibres, which in turn allows more containments through into the milk flow and the filter can split completely.
* A new filter costs approx 50c, whereas it takes at least $1 of labour to wash the filter (working on an average of 5 minutes per filter at an hourly rate of $12.)