In my opinion, only once but that view has been challenged by a record fluid designer who believes the process should be repeated semi-annually. Changing the inner sleeve at the time of steam cleaning ,as well as, encasing the LP cover in a poly clear cover reduces potential contamination . Of course, the air quality/purity in the home does affect the outcome as well. In my case I 've made the decision to have my home intensely cleaned : "Environmental Purification"; carpet deep extraction cleaning & sanitzing, dry cleaned drapes, all duet work extracted of dust and a house electronic filter. Cost about $ 500 - $ 1,500. I'm sure many feel that "over-kill" but that's a personal choice.