All you cat lovers - HELP!

I'm housesitting and I've got the feeling I've got the two most mischevious and cunning buggers around. Having never had a cat before I have some questions, maybe I'm just not doing things right:
1) How often do you do you change the kitty box?
2) How often do you do a complete kitty litter change?
3) What do you do when the cats pee on the kitchen floor (linolium thank god) and crap on the carpet? I cleaned both up and used a scent remove to get rid of the smell (something you're supposed to do with dogs, which are much more my speed), but want to make sure there isn't something else I should be doing.
Anything you've learned/heard/read would be great. I don't want the people I'm sitting for to come back to a week's worth of cat poop in their guest bedroom (or more importantly, me sleep in a room that smells like cat crap).
Thank you!
***
Its not all bad though, they have a crazy duck (whose name is 'duck'). She wants to be chased into the coop (which she shares with 2 chickens) at night. I mean she'll be in the coop but come rushing out as soon as you open the door go out and secure the coop. So you have to grab a bamboo stick and walk around after her. She'll run in a big circle and then run straight for the coop and get in.
Its the damnedest thing... Its like she can't go to sleep without being chased first. Crazy duck.

hmmm
Yeah, it could be that the litter box is too stinky and the cats are offended. Try cleaning it more often. Maybe empty it completely then try scooping out twice a day at least. Definitely the smell remover is key, because if cats smell urine or poop on the carpet they will start using the house as a bathroom. You might try closing some doors and only letting them in certain rooms to reduce their anxiety with their owners gone. I'm not sure what else to recommend. If the owners check in with you, ask if they've had this problem in the past.
cat poop
(1) Depending on the number of cats, you need to change the kitty litter once a day. I have two cats and one kitty litter box, and usually need to change the litter twice a day. The cats could be pooping/peeing everywhere because their litter box is too full for their taste.
(2) As for completely cleaning out the litter box and adding brand new litter, this completely depends on the type of litter they are using. I've read the grey, clay crap (which is the commonly used kitty litter) needs to be changed roughly once a week.
(3) Regarding how to clean cat pee/poop from carpet, I can't help you there (as we have hardwood floors, so accidents aren't as big a deal as with carpeting. I will see if Nat knows, as she used to have cats with carpeting.
Have fun with that Duck!