The answer to 'frequency of mulch' is that most experts recommend applying a 2-4 inch layer of mulch around your plants and trees 1-2 times per year. Applying mulch in early spring and late fall is ideal to help retain moisture, suppress weeds, and insulate plant roots from temperature extremes.
Mulch should be replenished when it starts to decompose and the layer becomes thinner. The exact frequency will depend on factors like climate, type of mulch, and rate of decomposition, but most homeowners find that reapplying mulch every 6-12 months keeps their landscape looking neat and healthy.