The best way to clean a reusable K-Cup coffee filter is to soak it in a solution of water and white vinegar, according to The Huffington Post. This process takes about 35 minutes.
To clean a reusable K-Cup, remove it from the coffee machine and dump out the grounds. Rinse it under running water to dislodge stuck particles, and then set it in a large bowl. Fill the bowl with one part white vinegar and two parts water, and soak the filter for 30 minutes.
Lift the filter out of the soaking solution, and rinse it with warm water. Let the filter dry before putting it back in the coffee machine.