It's all about RNG.

You can get 1/2 frags every 60 pulls, but since all SR could be obtained in a range of pulls, it's hard to say.

For my direct experience (pieced hashirama 2 weeks ago) i used around 2.7k scrolls and i got a kurama dupe (3rd SR) around 40-50 scrolls before finishing it.

Memo instead pieced him with less scrolls but didn't get the 3rd SR untill he went way past the 3k seals mark.