Thanks for answers. So, does anybody has real experience? According to old literature, it is said the nocking point and timber hitch are the weak points. But I have not found any numbers is it 1000 or 5000 or more shots? One hint is from Forbes (1955) he used linen strings, shot a lot and wore out three string per season. Active target shooter is shooting maybe 1500030000 shot per season, so it would be then 500010000 shots per string.
Marc I think that with natural material strings the strength of the string relation to the bow does not carry much significance. The string should has breaking strength 47 times compared to the bow weight. Old target shooting literature (Gordon, Elmer, etc.) says 67 times. It can not be much more because then the string would be too thick. The string wears out at nocking point and timber hitch, and relative strength has (almost) nothing to do with these points, supposing that those are somehow reinforced. But, I am not sure, I am just guessing, this would be nice thing to test.
So, I made one test string. Made a two loop linen string for modern fast longbow, 41#@28 not a easy bow for the string. Old Irish linen, 22 strand, breaking strength 280 pound, so strength coefficient is seven. Cotton servings, beeswax, mass was 130 grains.
Nocking point was too tight and it was causing some problems. After 3000 shots I opened the center serving and found that two strands was broken and two other strands was almost broken. Lesson learned
I made another linen string, almost identical but thinner center serving. Thousand shots so far and the string is like new. Hoping that it will last at least 5000 shots, maybe more.
Here is some pictures of the 3000 shots string. The last image is where you can see broken strands just under the nocking point (center serving is removed).