I would make cleaning a monthly event. Cleaning anything on the regular is always best (meters, equipment, containers, ect.). When cleaning the meter try to use distilled water so that you aren’t contaminating the measurement tool while cleaning. Most meter manufacturers sell cleaning tools and solutions for their equipment. You can do a 1/10 alcohol or bleach solution to clean. Do not over rub the contact on the meter- glass, silicone, and metal probes are delicate!