it can take years. The exception is if that time is that the runner is very talented but the time result of running inexperience, runner was completely out of shape or a it was a first effort time.

my daughter works very hard and she has improved about 30 to 45 seconds a year on the 5K, she started at 23 minutes as a freshman and is now at 21 as a Junior, she is the 8th fastest on her girls team. most girls in front of her have stayed in front of her by similar margins, most have improved marginally and proportionally. for example, girls running around 20 minutes as freshman are Running 18:30, 19 minutes is juniors.

I have seen boys or girls knock a few minutes off the timer once they commit to training, it can happen, but is very rare for someone to improve by a large chunk at a time. 10-20 seconds per mile is a big improvement year over year…and these are kids running hundreds of miles per year.