This is a question that comes up a lot when talking to homeowners about fireplace maintenance. The first assumption would be to remove the ash between each fire, but the this is actually not the optimum timeframe.
A one inch layer of ash on the floor during heating season is actually what is recommended by the Chimney Safety Institute of America. The layer of ash works with the hot coals and creates more heat to fuel the fire, helping in a cleaner burn.
While around one inch is a good number to stick with, if the ash starts building up too much, it can trap hot coals between the fireplace grate and the ash, causing premature deterioration of the grate.
It is also the important that if you have a ash dump that it has been inspected and is free of combustibles. Also, do not dump your ash until it has fully cooled to prevent possible fire or even injury from burns.
In the Spring, remove all ash from the fireplace once you are done burning until the Fall. Ash can actually absorb moisture over time and the combination of ash and moisture creates a very acidic compound that can not only stink but prematurely damage your fireplace.
Yet another reason to have your chimney swept in the Spring, we can take care of all of the remaining ash during the service. Search for your local CSIA certified sweep here: