There is no standard frequency at which to clean an LCD TV screen, though the website Help With PC recommends cleaning around once a week. Some screens will require more or less cleaning, depending on environment and usage.
LCD screens become smudged and dusty just like the glass screens on cathode-ray-tube televisions. Households with large amounts of dust, children or pets may find that their screens become dirtier faster than those without these factors. Regardless of frequency, LCD screens are more delicate than glass counterparts, and should be cleaned with a dry microfiber cleaning cloth. For persistent dirt, experts recommend a solution of water and a very small amount of mild dish detergent.