It depends a lot on the terrain type - highly repetitive terrain requires higher overlap. It does sound like they have some broken setting somewhere though.
It's heavily pine forested mountain areas. With a 65-75% overlap, the SFM algorithms struggle to produce sufficient details. Additionally, because of the verticality of the terrain and very tall pine trees there's a need to have multiple angles to generate a good orthophoto. So the grid is denser than other environments for a reason. I'm continuously updating my flight plan based on the results generated -- squeezing density up/down based on observed results.
The 4 hours is an overestimate, it's probably genuinely closer to 3 hours flight time.
Just curious, why do you have to survey repeatedly? Are you monitoring something changing?
The area was partially clearcut about a decade back. Some areas are due for brush management and some for commercial thinning. Additionally, because it is alpine and contains a stream used by fish for spawning, it is interesting to see the variations in snow load and water flow in the stream year over year.
So there's at least a reason to get out each winter (snow load), spring (melt/brush growth/flowers), and summer/fall (stream health/identify trees once brush loses leaves).
I also like seeing if there's trees in stands dying at an unusual rate, which might indicate pine beetle infestations or sickness that I'd need to take care of.
Also, it's a fun hobby and a cool dataset to flip through.
I have a similar situation with a smaller plot of timberland. Sometimes even with 90% overlap it can’t stitch. I get your pain.