Shelley-Ann,
Keep a record of when you replace the seal.
That will help you decide when it will be time for a next fresh seal, without having a failed seal ruining photos alert you that that time has come and gone already.
Depending on your readiness to gamble, you will have to change seals every year or so. Once a year is perhaps overly cautious, but also very safe. Once every two years is already beginning to push it.
The foam ages and deteriorates all by itself, independent of use. But keeping it compressed by keeping the dark slide inserted will help speed it up a bit (or if it does not actually speed it up, it will help press the stuff into a permanent, flat state you need to have light leak in.)
So store empty magazines with the dark slide out.
I cut my own foams from a black closed cell foam sold in sheets in hobby/craft stores (it is sold here under the German name for the stuff: "Moosgummi". That would translate to a generic "foam rubber". I don't know what the particular stuff would be called in English.) Extremely cheap, but also extremely suited for the job. Being dirt cheap, and quick and easy to do, i change seals probably far too often.
I'm sure you can find the same, or similarly suited, black foam in crafts/hobby shops everywhere on the world. See if you can find any, and then start changing seals far too often too.
The original foam bits have a 'metalized' piece of plastic stuck to it, to give it a curve along its length. That curve makes a thin piece of foam fill a thicker gap, and provides some 'springiness' to allow room for the slide without compressing the foam too much (part of the pressure is taken up by flattening this plastic blade spring, instead of compressing the foam.)
Any replacement foam will not have that spring backing and needs to be thick enough to fill the gap by itself. Which is not a problem; you can get foam in many 'thicknesses'. The foam will then be more compressed when the dark slide is in, compared to the original. And there will be a bit more resistance when you insert the dark slide. But apart from perhaps ageing a bit quicker (no problem: just change seals more often), i can't see any disadvantage in that.
As i said before, you only need to change the foam. The foil that is folded over the foam only needs replacing when it is torn. So reuse it until it does show signs of imminent failure. (The foil serves to protect the foam against the sharp edge of the dark slide. Nothing more.)
I haven't found a suitable (and cheap) replacement for that foil material yet. I forgot who it was, but on page 1 of this thread someone mentioned using a bit of film cut to size.
Oh, and to give you an idea about the savings: i'm not sure what original seals go for now, but assume it's in the $10 - $20 range.
One A4 sheet of that Moosgummi costs about $2, and is good for an awfully large amount of foam pads.
