I have a long time without using my Jato and I recently have been using it but I don't remember when should I replace the glow plug, when should I replace it?
The best way to tell is to take it out and hook up the blue plug wire, hold the side of the plug against the side of the head and push the easy start button. Or use an igniter. If it glows good and bright, it's good. Also look for element damage, like being deformed or burnt looking. I always keep at least one new spare, you never know when they'll go bad.
You normally replace them when the stop glowing you will no cause it wont start. If you want to visually check it pull it and if the glow wire coming out of the center is gone well you need a new one also if you flood (foul) it change it until it dries and clean it.
JATO, MERV, 4-TEC, & NITRO-VEE
If you start it and the engine will kill when you remove the power source to the glow plug. It could glow all day with a battery hooked up to it but if it will not stay lit in the presence of the fuel then throw a new one in there.
The Super Derecho
If you got a glow ignitor, simply press it in like you do when you start the car and see if it lights up. If it glows then its' fine as far as ignition.