I think the rule of thumb my wife used to use was 65 inches for a 5 decade rosary (#9 cord with 6-8mm beads and no intra-decade spacer knots). I remember she’d basically stand on the end and then pull it to roughly the top of her head. Shes 5 ft, 5 in so 65 inches. It seemed we used maybe a foot for the tail end, so 53 inches for the 5 decades.
If you round that up to 11 inches per decade you might be looking at say 177 inches of cord for 15 decades and the tail? That is probably a bit extra since you only have to make 2 loops to connect the center instead of the 6 you’d get by tripling the length. It’s only a couple extra inches, but …
If you are using thicker cord and larger beads it will take a bit more. I can’t remember off the top of my head, but the next size cord (#21 ?) we needed maybe an extra 6 or 8 inches?
Sorry, trying to dredge up memories from 6 - 8 years ago when we would sit on the couch making cord rosaries by the dozens.