It developed over one summer but it only bothered me when wearing spikes at first. I put it down to buying sprint spikes instead of distance ones, which was a big mistake, for 400/800, it was stupid.
At first it only bothered me during track sessions but it got worse and worse, I ran the last race of the season in August and it was bad after then. I was in a lot of pain walking as the other person describes. So I think I ran on it for a complete summer. It then took about a year to clear.
If it helps, I did get what I consider full and complete recovery. However, due to a different more traumatic injury, I had to give up running shortly after, so I don't know if that played a role. From what I remember, it started to clear up after about 3 months of complete rest. The very worst was in August, I remember being annoyed at having to take 'two weeks off' after the track season. I'd give anything now to hear I could run after two weeks. I think by December time it was a LOT better.
All of the advice I was given was utter rubbish for me at least. The guy I got advice from was studying this area for a PhD, he was adamant rest was the worst thing for it and you had to do the calf raises/drops to have any chance of recovery. And to do this thing were you walked in progressively flatter shoes. It aggravated it so much. It used to 'pull' when I walked, every step, when my foot was behind me. It was when I stopped doing all of that it started to get better.
The washing up bowl with warm, not hot water really seemed to help. Ice made it a lot worse. Even cold water made it bad at one point, it would like creak if I tried to walk after having my foot in cold water.