The dust mite covers on the mattress only need to be washed once or twice a year unless something is spilled on them. If the covers are for a child and the child still has accidents every so often then the mattress covers will need to be washed as needed.
We wash our pillow covers about every 4-6 weeks. Just put them in the weekly load of sheets.
For additional information and care instructions check out "How to Care for Your New Mattress Covers" in our resource center.