These time criteria are kinda garbage--different people respond so differently to various types of training that there's nothing you're really gonna be able to predict from having an average guy run 25 mpw for 4 months. Plenty of guys who could run, say, 35 minutes for 10k off 25 mpw for 4 months could totally crumple under medium mileage and never really get much faster than 32ish minutes. Plenty of guys who couldn't break 40 off of 4 months at 25 mpw could end up running sub 30 after a decade of careful training and buildup to high mileage.
The best (and probably only real) way to discover a runner's potential is to have him/her train carefully and consistently for the better part of his/her youth, adolescent, and young adult life and then see how fast he/she is. You can make vague predictions based on how quickly someone improves, but any real attempt at finding a time range for them is useless.