Pretty much after every trip.

Cleaned my cat last night .... what a chore!

Should be every time you put it in or take it out of the water.

Think Zebra Mussels and Salvinia.

I don't clean mine as much as I should but when I think about it, I take a scouring pad which is on a telescoping handle, and/or a stiff bristle brush on a long handle (mop size)and just dip it in the river and clean the boat while we fish.
It sure makes cleaning later on a lot easier as those shad guts and skipjack blood come off then and don't dry and cake on for later.
As for a big cleaning, I do it about 3-4 times a year.
A big bucket of Fabuloso and elbow grease with a stiff bristle brush. Rinse the heck out of it especially if it sprays on the livewells.

before every duck season. seems like a good time when I change it over from fishing to hunting.

Once a year or when the boat trailer tires begain to show stress.

I don't clean mine every trip, but probably should. Usually after 2 or 3 trips, because it starts looking nasty and the floor drains get clogged up.