The string you mention should be fine for back up, no problem!
I change mine when they start looking bad but I have never worn one out. I make my own strings which makes it a lot more cost effective to change every year or two though. I should say also that I have some that have been on a bow for several years with a lot of use and still look fine.
Bottom line is if you're break a string there might be other things at play causing the problem. The new string materials are pretty bomb proof.
I agree with this. The only time I’ve had a string break was a six strand string. I think the fad for ultra skinny strings has come and gone, but I’m sure there are a lot of them still out there. The six strand string I had was rated for the draw weight of the bow I had it on, but I don’t think it had much reserve capacity. Probably one strand broke, and then the rest of them went along with it. I make all my own strings now too, and I make 14 strand strings. 14 strands is more than enough for the weight I pull, and that’s what I want: more than enough. I inspect my strings frequently, and I want a string that if I find a strand or so frayed, or the end loop frayed from friction against the string nocks, it will give me some warning before it breaks.