If you're superbly talented, and I mean more talented than pretty much anyone, the top .0001%, you might reach his level at 80000 plays. Which if you estimate with 2 minutes per play (a fairly low number), is like 2666.6 hours. 2666/12 = 222 days. This is assuming you play 12 hours a day straight. Summer would be over by then. So unless you're playing more than 12 hours a day, it's actually physically impossible to reach his level by the end of summer. The estimates I gave you are assuming you're unbelievably talented and incredibly motivated.
Assuming a more normal level of talent, still much above average, I'd assume you'd need about 110000 plays with a song average length of 2.5 minutes. This would result in requiring 4583 hours to reach his level of skill. Let's say that you can play really hard for 8 hours a day, that would take 573 days of intense training. This is probably a much more realistic number, and even then, you'd have to be super disciplined to play this much everyday along with finishing school/work/etc.
So basically, maybe if you're 12 years old and you train 10+ hours a day for the next 8 years. At that point, you'll likely be as good as Cookiezi if not better.
If you're already mid 20's, it's unlikely you'll ever reach his level, as reflexes slow down, etc.
Within 15 days, my rank has changed massively, but even still I don't know if I'd even reach the top 200 (they're already immensely good, but it is something that's quite possible to achieve if you put in massive amounts of time).
Saying that you'd read cookiezi's level of skill within a year is just plain ignorant. It's like saying you'd become the best in the world at something with only 1 year of hard work. There are many people that are already at a godlike level of skill that can't reach his level of skill even though they've probably played single maps more than you have played osu.