We've left our cat for 5-6 days before without an issue. We usually leave a huge bowl of dry food and one full can of wet food (she doesn't seem to just gobble it up in one sitting and get sick like some animals might) and several bowls of water. We also leave all of our toilet seats up (just in case) and make sure that the temperature is comfortable (heat or air conditioning on- depending on the season).
One caveat, make sure you actually see you pet before you leave and then don't do anything else. One year, we left our cat and shut all of the bedroom doors (we had someone coming in to feed her because we were gone for 10 days). I ran back up to get something at the last minute and she must have slipped into my bedroom without my noticing. Anyway, it turns out I shut her into the bedroom accidentally.
It was a good thing that we had someone checking on her who noticed that she hadn't eaten any of her food or used the cat litter. He went looking and found her in our bedroom looking a bit dehydrated, but fine. He led her to the food/water and she recovered just fine. However, it would have been a different story if he hadn't checked in.
Moral of my story- Actually see your pet...and then leave!