It would seem to me, that your glow plugs, even up north, should last for years. If I remember correctly, someone posted on this forum that he tried Ford diesel plugs, rather than Mercedes (Bosch) replacements. The Bosch were his originals and had been in the car for 16 years. His Ford replacements lasted 2 years and failed.

It seems like there's something else wrong with your car, perhaps, if they don't last at least 5-10 years, even in your cold temps. The other thing, is that someone posted a YouTube video, where he starts his car in 0 degree weather. I think he cycled his plugs only once, but the car started right up. YouTube it and see.

I guess in your cold weather, at 0 degrees, you need to add something to your fuel so it will be thin enough to start.

We don't get that cold here in Atlanta, but I've no problems whatsoever, of starting my car on 12 degree days. Starts up just like it always does, and purrs like a kitten, though it takes the plugs maybe 15 seconds longer to warm up before I can turn the key. I've never cycled them twice, ever.

1991 300d, 166k