make a float variable called coilHeat. Every time you shoot the gun(wherever that event is in your blueprint), add a number to coilHeat and set the new value to that addition. Then add a branch that checks if coilHeat is less than 100 (or whatever number you want) and then set a new boolean variable called Overheated to true.
In event tick, use a branch to see if coilHeat is less than or equal to 0. If it is, subtract the delta seconds pin from coil heat (you can multiply deltaSeconds to make it subtract faster)