How big is the jump from 17:40 to sub 17?

Less than 14 seconds per mile or 3.5 seconds per lap if you go down to your local track.

It depends on your background and personal info. If you are a high school boy it might be easy to do in a year. If you are an adult it would be easy if you work up to 70mpw

Everyone has a different ceiling. That said if you went from 18:15 to 17:40 off of modest mileage in 3 months it's unlikely you have topped out. Keep training in the 35 to 45 M/wk range and you'll get faster. Sub 17:00 is a reasonable goal

Most HS/ college age males with reasonable talent don't need to bump their mileage above 50/wk until they get in the sub 16:00 range.

After that, you have to start dropping your mile time below 4:30 and/or significantly increasing your mileage to keep cutting time. Good luck-

It's about 4%...

when I was in high school, I'll tell you what held me back, and how I held myself back, from breaking 17:00 -

1. life outside of running in disarray

2. poor, inconsistent, or insufficient sleep

3. junk food diet for more than 50% of my calories

4. never socializing and instead spending 4 hours a day sitting playing MMO whenever I could

5. never questioning my training... i.e., to me, easy moderate length to long runs felt as if I had to run them at a 7:00 pace or faster, even though I probably shouldn't have gone near or under 7:00

6. Bad coaching- two years of my high school, the coaching torch was passed off to a lazy AAU recruiter and personal coach who would prioritize those clients over my school's small student body, and another year, stand-in and assistant coaching duties were passed off to a teacher who really had no experience in cross country and was too fat to come close to keeping up, but he at least tried to coach us well, using his bike to keep up. Private college prep program problems, I guess?

7. injuries, excuses - during the years of bad coaching, as well as all of my other bad diet and lifestyle habits, I got stress reactions and ITB fasciitis. I could have avoided this stuff by gaining more knowledge and better recovery.

I now know I have a propensity to get stress reactions, but didn't back then. I now know that my calcium intake sometimes flags, which I most recently experienced in my 20s when I had basically hand arthritis which went away within 2 weeks.

In my 16th year of life, my stress reaction of the left foot could have healed twice as fast or have been entirely avoided. The AAU coach had our distance guys run speed intervals always counterclockwise and for 80% of our daily practices around the football field, which made repetitive stress injuries. This coach also gave no real prescription for slow running, it was basically get in and hammer speed intervals at an arbitrary fast pace.

8. lack of camaraderie, laziness. This harkens back and ties in to my earlier point about not being social and spending hours in computer land. My first two years of running were immaculate and joyful - my small NCISAA school had a renaissance in 2006-2010 which ended when the groundswell of talent and good coaching ran dry. I got depressed, the program faltered, and being a socially inept dummy and having my older brother and his peers who had led the team graduate, I didn't pick up the slack to bear team leadership. I rarely ran with anyone or did substantive runs outside of practice or out of season.

9. Not playing another sport as cross-training, not lifting weights. These yield confidence, fun, motor skills, recovery, fitness, and injury prevention. I was intimidated by not being good enough, and I hadn't learned to embrace my weaknesses yet

Just over 40 seconds.

If you are still new to running as you state and are improving fast, this will not take much time. Would guess like 5months
For me and many friends improvements came in abput 40-60s every 4-6months down to about mid low 15s, starting at 21-20min. In the beginnig even faster like 2-3min in the first 6months.

If you've already run 17:40 off of relatively low mileage then sub-17:00 is a very attainable goal. I don't think sub-17 is as tied to genetics as sub-16 & beyond are. If someone is healthy & able to log consistent mileage then it's on the table. Build up the mileage a little more & keep progressing through a lot of the same workouts you've been doing.