Someone who is very talented and not in terrible shape to begin with could run sub 2:30 off only 50 miles per week. Someone else may require 90 miles per week. Someone running 120 miles per week may run 2:09, someone else may only be able to run 2:35. It's different for everyone, but in general what ever you had to do to run 2:30, you've got to do a little more or do it a little better to run 2:25, and even better for 2:20.
If you're not too old, if you do the same thing you will actually probably keep getting slightly faster, but if you did slightly more you'd get even more faster.
But just for the sake of argument, most people probably aren't going to run under 2:30 unless they're running at least 80 miles per week. And most people aren't going to run under 2:20 unless they're running at least 100 miles per week. For women most won't run under 2:30 no matter how much they run per week.