Simulink "stateful" subsystem

I want to create a stateful subsystem in Simulink. Meaning: I currently have a subsystem which performs a calculation on its input for every step of the simulation, but I want the subsystem to have a state, meaning that the calculation is made every X iterations according some output which was calculated in previous iteration.

To be more specific: I currently have a subsystem which is a MATLAB function that represents an Air-Conditioning system. The subsystem gets the environment's temperature as an input. Its output should be the A/C temperature. Since the A/C temp doesn't change in an instant, I want it to have some slope - that the change in temp will occur over few iterations of the simulation, and even though this is a closed feedback loop, I don't want the subsystem to recalculate the desired A/C temp on the following iterations, until the desired temp is reached. Once a desired temp was reached (which could take 5 or 6 steps) then the subsystem should recalculate its output.

Is there an easy way to accomplish this?