How often to replace thermostatic mixing valve
If it's an adjustable valve, some times fully opening and closing on a regular basis can help keep the guts free. After that attach a thermometer to the tempered water pipe, try running some Luke warm water at a sink while adjusting the valve, if temperature changes do not correspond with changing settings the valve is faulty. I would have a replacement on hand prior to disassembly, sometimes these things don't go back together nicely.