Dear Mr./Ms. Bossname
After (amount of time) as a (position) here at (company), it is with great regret that I tender my resignation, effective (date). Although I've been here so long that it feels like a second home, I believe it's time that I (move to a new location, spend more time with my family, travel, take care of my health, etc.)
I hope that my (number) years working at (company) serve as a testament to my dedication to this company. I have enjoyed being a part of a team that (brief list of accomplishments) and I am grateful for all the opportunities to contribute to (field).
Thank you for everything you have done for me over the years. I wish you all the luck with your future.