SUBREDDIT: r/jobs
TITLE: Is 8 1/2 months notice too much?
POST: I've decided that the end of this calendar year will be the optimal time for me to quit my job and return to school to pursue another degree as a full time student. I've been here for about 2 years as of this week. I've already breached the subject informally with my direct supervisor, sharing my desires to return to school and transition to a different career field.

My question exists as follows: Will I be doing myself a disservice by announcing my departure so early? I feel like it would be optimal for my employer to know as soon as possible so they can arrange for my replacement and see that the projects I'm currently managing are properly closed out or transitioned. I also feel like I may be jumping the gun and giving off the impression that I can't wait to get out etc?
TL;DR: 
I'm going to resign my job in 8 1/2 months. Do I drop a formal letter now or wait until December to make my departure official?