Depends massively on your age, current training, training history etc.
I've been running since I was 13 and when younger was always useless at the shorter mid distance events but much better at the longer stuff. At 16 had pb's of 2.06, 4.09, and 8.57. The next year after another identical winter of training (basicall 30ish mins of fast steady running after school Monday - Friday with a cross race most weekends) I went 1.57, 4.02, 8.40. Still cant quite understand how its possible to drop 9s over 800 and 17 over 3K but only 7 in the 1500. Next year I dropped again to 1.55, 3.52, but only 8.40 again, still doing the same training as before. Probably important to point out that I significantly 'filled out' over this couple of years. Over the years I've seen similar things happen with a lot of younger athletes I've helped out.