How often do zombies bite plants and how many bites can normal plants take? I know Wall-nut and Pumpkin take 72 bites and Tall-nut takes 144 and Snow Peas reduce attack speed by a magnitude of 2.
According to this thread, different zombies have different bite speeds, and consequently may bite plants different numbers of times. However, even though they bite faster, zombies should all still take the same amount of time to eat through a plant (barring the exceptions outlined below).
I found a list of health for all plants here in terms of number of zombie bites until they are destroyed.
Note that "normal" in this table means:
6 or 4 bites from zombies, 1 hit from vehicles or Gargantuar, or 16 peas from ZomBotany zombies. All plants can only take 1 hit from vehicles or Gargantuar except Spikerock taking 9 hits.
As such, the default amount of bites to get through a plant appears to be 6. However, as you mentioned, some exceptions exist, including Wall-Nuts and Pumpkins taking 72 bites, Tall-Nuts taking 144, and other plants like the Hypno-Shroom taking 1 bite or Garlic taking 21 bites. Other exceptions can be found in the list.