I've heard it takes roughly 3,000 repetitions to really make something stick in muscle memory.
Obviously we get better long before the point, and 100 perfect reps are better than 3000 bad ones, but more is better.

RE: Stone throws. I actually competed in Bakersfield over the weekend, and while I didn't get any pics and I only took fourth, I did PR in Braemar Stone. The key was my elbow. I had not been throwing my elbow of the offhand back like I was trying to knock someone out before. I did it in Bakersfield this weekend, and two of three throws set PR's for me.

I'll be out on the practice field again this weekend, putting in more work on technique and building those muscle memories.