TLDR: Around 2,000 skins before I found a singular Pristine Leather.
I leveled a rogue to 20 and maxed out skinning. Never saw a single Pristine Leather while playing it. I did NOT have Bountiful Harvest on this character. I also don’t have any specific stats for this character because I didn’t have any reason to track it at the time.
I leveled up a hunter originally with engineering/LW and swapped into Skinning when I noticed basically everything requires Pristine Leather so I couldn’t craft most things.
I watched a couple movies and skinned roughly 1,100 mobs on my hunter starting at level 25. This character DOES have Bountiful Harvest I didn’t start recording drops/kills until around the 500 mark. (I finished my first movie and still hadn’t gotten a Pristine Leather) Almost exclusively Ghostpaws, Ashenvale Bears and Shadowhorn Stags (levels 23-26) until I hit 27 then I moved onto Ghostpaw Alphas and Elder Stags (26-28)
I got 4x Pristine Hide on my my 207th and 208th recorded kill off of 2 level 28 Ghostpaw Alphas (2x each)
After getting Pristine Hide I assumed that meant level 28+ mobs might be too high level to drop Pristine Leather. I then moved into Southern Barrens and skinned Hecklefangs Stalkers, Stormhide and Greater Thunderhawks. I killed 304 mobs between the 3 of them and did not get a single Pristine Leather. I also did not find any Pristine Leather while leveling up skinning on this character.
I then decided I wanted to try a shaman and was now committed to finding my FIRST pristine leather. Again without Bountiful Harvest. At level 19 I sat around farming in Redridge and bonked Boars and Whelps. (85 of them total) and hit 20 and went back to Barrens. A combined 204 Thunderheads, Stormsnouts, Sunscale Raptors, Zhevras and Giraffes later I obtained my FIRST Pristine Leather from a level 19 Stormsnout.
This genuinely can not be the intended drop rate. Account wide I’ve had to have skinned between 1,800 and 2,000 mobs to find a singular Pristine Leather.
I did have around 20 mobs overall that I skinned and had absolutely nothing when skinned.