That's a good question and one I tested out in the late 1950s. For some reason I can't explain, I got interested in knots about that time (the Practical Fishing Knots book was a result of that). I tied 90 blood knots in two different diameters of monofilament. I broke 30 of them immediately and recorded the break strength. Six weeks later I broke 30 more. Then a year later I broke the remaining 30. I was most surprised to find there was less than 1 percent difference in the knot strength between those broken immediately and those a year later. Of course, I was careful to properly store the knots to be tested at a later date.