We have seen a lot of cases since the drought started here some 3 or 4 years ago. If you treat it aggresively, it takes about a month to completely clear up. If you treat it less aggressively, it takes about 30 days to clear up. If you just leave it alone and make sure the horse is eating and drinking well, it takes about 4 weeks to clear up.
The last 3 or 4 cases we have had I just left out in pasture and checked on them every day. None of the other horses got it that they were running with them and they were fine in 30 days.
Like everyone else, we thought the first one had been kicked. Of course, we isolated and treated the 1st case aggressively when we found the horse had a fever. We left the pen empty for months, disinfected everything that had been near the horse. The whole deal.
The next year we had one get it. We put it up but just watched it closely, flushed the abscesses when they broke and put the horse back out. The last one I did not even bring out of pasture. I just watched her very closely. None of her pasture mates got it. She did not get drawn up or dehydrated, so I did not even bring her in.
How to tell the difference? Take the horse's temperature every day. A hematoma will not usually elevate the temp unless it abscesses (which one can). Pigeon Fever will elevate the horse's temp, usually significantly, before the abscess breaks.