how much can you improve in one year?

Not sure a lot of factors play into how much you improve

genetic potential
technique expertise
growth
desire
other training
weight gain

Looking at this youtube the kid looks very young and inexperienced in first shot and a year later looks quite good plus he looks like he grew a bit. Never under estimate growth and its effect on distance. I remember when I was 17 and i first threw the shot i think i threw 11m something and a year later without any training improved over 2 metres then after a year of training I improved another 2.5 metres.

One of the 17 year old girls that I coach went from 12m to 14.80m in 11 months just increasing her training from 1 session a week to 3. This was purely technique and throwing strength gains that lead to her improvement. I cant wait til she hits the weightroom

hope this helps

I've been coaching a very long time and I went back and the greatest freshman to sophomore year improvements I've seen were 17' ( 29' to 46' ) from a kid who went 62' as a senior this year and 16' ( 32' to 48' ) from a sophomore this year. Along with the 10,000 throw concept I always tell beginners it takes 1000 throws before you'll feel fluid and natural. Almost everything about the discus is unnatural. Upper body staying wound as lower body gets ahead, keeping the body in the middle after driving across ring instead of pitching to front etc.