Personally speaking, pz starts to get way too easy when you hit around 500ish hours. And the sole reason for me it's the loot situation in towns. I think that the levels of loot in the first 2-5 days, should be pretty high, (due to the fact the most people would rush to the stores, thus making them infected) but afterwards it should start declining rapidly, so after a week or two, the stores would have very little loot left. House loot should decline but not in every single house but just in the houses that other survivors have already looted. Also the same things as the store situation should apply to EMS buildings (especially the police), so that EMS gear and tools could only be find on zombies. This should only apply to cities and not to isolated places!