I will not pretend to know anything definitive about fruit as I am a complete noob. However, I have collected close to 2000 in the two weeks I have played. I had about 200 last night in 2.5 hours of walking. It was a troika of small (~25) finds of bombardo, haimoros and papplon then a big (121) papplon. That seems about average for me, plus some dung (I have nearly 1500 dung now, anyone want some?).
I basically walk everywhere, Amethera for the most part, and besides occasionally having to revive I almost never see another person. Getting out of the traffic lanes seems to help. I have been mapping my finds, but I've not seen anything repeat exactly yet, though I suspect that things may spawn in certain general areas. And stuff does spawn under water, I had three finds the other night, including a 100+ papplon underwater.
I am still trying to figure out the right patterns, I have walked long stretches of the world for hours on end and not found anything then I will walk another area and have two finds within a couple minutes. It's an exercise for the very patient, but seeing the world, collecting TPs and not having any PEDs are all good motivators.
As for theories, I haven't fired a single shot in my entire fruit-finding career, so take that for what it's worth and I frequently find fruit in places where there are no mobs around for ages.