I retire in 2021 and am considering a career as a JROTC instructor. Are there any JROTC instructors out there who can provide some advice?

Yes, I'm Navy, but I was in an Army JROTC (cadet XO), and while I was working on my Masters I substituted in my old high school, including a stint as a long-term sub in that same JROTC program. Our Army JROTC program had 2 back-to-back national color guard champs, multiple state drill team champs, 2 state raider champs, 2 state academic champs, and is in the top 5% of all JROTCs (Honor Unit with Distinction).
1) Do it
2) According to my latest intel (about 3 months old) you can't have been out longer than 5 years before teaching JROTC
3) It's not about making future soldiers; it's about making better American citizens
4) Federal AND state retirement? Heck yeah! Converse: Army and Principal bosses, well, most Principals assume you know what you're doing, but the Army will do an IG inspection every 4 years.
5) You'll get to speak into students' lives where no one else can.
6) Start looking now! An empty spot won't last long, and as mentioned elsewhere, people stay in a spot FOREVER.
7) You're not a soldier, you're a teacher. A teacher who's been a soldier. The kids aren't soldiers, they're to learn the trade of being a soldier IN ORDER TO BE a better American Citizen. The map reading, marching, first aid, military history, staff work, shooting, rappelling, chain of command, all that is to make them productive to this society. If they end up joining, great!
8) Talk to other JROTC guys, e-mail them (most school websites have faculty's e-mail). Network!
9) The Army can afford to be picky right now; the Major in my local unit told of a CSM who'd been arrested when he was a CPL, the charges were dropped, and 30 years later made CSM. Did JROTC instructor, 10 years, and when they did the background check to work with kids, this arrest from 40-ish years ago, they dropped him.
10) Do it! Touch the next generation, Lord knows they need an example of honor and patriotism.
Best of luck!

I have gone through their website and obtained as much information as I could. How long can one be an instructor?

I am not a JROTC Instructor, partly because there are not a lot of programs near me (7 in a three hour radius, 1 within 1 hour) and guys hang in there forever. If this is your goal watch the open positions and choose your retirement location wisely.