I am not aware of a standardized schedule for applying mulch. It is more often when man power and the mulch is available through the landscape maintenance firm. There are some important points to review with the landscaper so your funds are well spent. The mulch should be no deeper than 2-4 inches once settled. Before new mulch is added to existing mulch, the old mulch should be broken up and loosened to avoid water penetration problems through the mulch. If the tree ring is to be renewed/freshened, the soil should be removed off site or used properly in other areas to avoid excessive build up of mulch and soil in the tree ring itself. The soil will actually hasten the decomposition of the mulch. Mulch should never touch the trunk.