The wax appears to be due to incomplete combustion of fuel gas. My suggestion is to have the wax analyzed for chemical composition. This will identify the cause of the wax buildup. Then, depending on the wax composition, the most probable solution appears to be a redesign of the burner tips.

The rate of buildup can only be calculated by looking at the amount of buildup during each cleanout and then forecasting it for the future. Obviously, there will be a maximum limit as to how much wax will accumulate, then there will be numerous other problems.