-is it necessary to replace glow plugs before they fail? as routine maintenance?
-do they have a lifespan? depending on use, number of starts? or just randomly fail?
-best before date? etc..
-if they have resistance measured with an ohmmeter, is that good enough? or should it meet a certain level?
-just wondering..

Typical resistance is 0.1 to 6 ohms but may still be good up to 11 ohms.
Kind of a random failure situation.

Here are some signs of damage..

We always replaced them when reconditioning the head or around 10-12,000 service hours. We would inspect them at every 2000 hrs when adjusting valve tappet clearance.

a point to remember when checking the resistance, it should be read quickly ie a few seconds not half a minute. as resistance changes with temperature. also you dont want to burn out your multimeter. also buy quality units they last better. a light dab of grease or copper anti-seize on the thread will aid install and future replacement.

There is an inch lb torque rating for glow plugs also, check in your engine service manual. 120+-24 INCH lbs for plugs on 3304,6 engines

My experience with glow plugs has been more extensive and expensive with Ford 7.3 IDI engines than "cold blooded Cat motors" as my Pap would call them, but I expect it's all similar. An engine will crank fine with them until it doesn't. Then one or all may be foul. Good steady voltage and clean tight connections are never a bad thing. And good quality replacements are sometimes a bargain at twice the price. Anyone who has been lucky enough to wiggle a Autocraft, Beru, insert generic part name here plug with a swollen tip out of those old IDI engines will probably argree with me. Also, some of the better manufacturers will give you resistance specs that their parts should fit.

But, as far as prevention of the problem goes, I reckon if you're dealing with a tired engine with more hours than compression or suspect fuel issues, not within your engine but within the cetane in the tank it's probably not a bad idea to add them to the maintenance list. We've done that a time or two on Pap's ol F250. And more recently I've watched the way the machines on the job have cranked on our sometimes questionable fuel. All different brands and different "Tiers" as they like to label them. It will certainly make you wonder.

trainzkid88 wrote:

a point to remember when checking the resistance, it should be read quickly ie a few seconds not half a minute. as resistance changes with temperature. also you dont want to burn out your multimeter. also buy quality units they last better. a light dab of grease or copper anti-seize on the thread will aid install and future replacement.
 

Never heard of burning out a multimeter checking resistance for too long. You will do damage if you test resistance in a powered circuit. Most multimeters only put a small voltage through the circuit to test resistance and it shouldn't be enough to cause a glow plug to even get hot.
In the past I have connected alligator clips to my multimeter test leads and left them connected checking resistance for up to an hour with no side effects.
Agree with the comment about a quality meter though. I learned that in Auto TAFE and only use Fluke.

In regards to the thread questions; personally I think you would be throwing away perfectly good glow plugs if you replace them periodically. If you have an amp meter installed you can then tell when they are not working.