This is far from a stupid question. Most people do not maintain their interiors properly.

First of all, your interior lasts based on how you treat it. Without any cleaning/conditioning products, the leather and the interior will certainly last for most of the life of the car, assuming you treat your interior well (you don't have to baby it, just don't abuse it).

Let me answer your question by telling you what I personally do to maintain my G's interior. All of my procedures are not requirements at all...more like steps I take to keep my interior looking fantastic and germ-free.

I do this whole "Interior Detail" process once every 8 weeks:
1.) Clean the leather with Lexol Leather Cleaner. I clean everything in the car that is leather (using Lexol). The leather seats all around, the headrests, the steering wheel and shift ****, and even the center console compartment cover thing.
2.) Condition the leather with Lexol Leather Conditioner. I condition all the leather I have just cleaned with the Lexol Leather Cleaner.
Note: I used to use Zaino Z-9 and Zaino Z-10 but I find the Lexol to work much better with our car's leather.
3.) Shampoo the carpets. Basically any carpet cleaner form KMart or Target or something will get the job done fine. Take the carpet mats out of the car, spray the stuff onto the carpets, and scrub with a hard-bristle brush. Then let the carpets air-dry.
4.) Wipe down the rest of the interior with Einszett Cockpit Premium. This stuff is good because you can spray it on ANY surface (even your paint) and it cleans well and dries completely streak-free, and has an anti-dust formula that keeps dust off for about a month. I use a regular towel and just spray the whole interior at sections at a time. I clean the whole dash, the center console, the insides of the doors, the door sills, etc. (even the pedals!!).
5.) Vaccuum any loose dirt/crap lying around the floor...those of us who live in the snow belt will know what I'm talking about !!

You may also clean the headliner with a headliner cleaning product (obviously, hehe) but this is one part of the car where I'm lazy. Headliner material keeps dust away, does not get sun damaged, and does not fade or anything over time. Since I personally never touch the headliner, I figure it stays pretty clean and I will not benefit by performing any maintenance.

I should actually write up an interior detail process for this forum. Not that what I do is right, but my methods are proven.

dennis_said, I actually should have specified. My navi display also gets mad dusty. I just gently spray the Einszett Cockpit Premium once onto the towel and gently wipe the screen until all the dust is gone. Make sure the screen/car is turned OFF when you do this and apply very little pressure to the screen. Keep in mind that all you're doing is picking up the dust off the screen (you're not really "wiping it down" like the rest of the interior) and like all LCD screens, you can damage it if you are abusive.

G35Tuning, it is ultimately your call. If nobody really rides in the back, you can safely leave it alone without any real difference. But if you plan to own your car for 5+ years, you don't want your front seats looking in great shape and find your back seats looking a bit more worn down. Personally, people sometimes do ride in the back seat of my car and since I am going to clean and condition the front seats and steering wheel, etc. anyways, I figure I might as well also do the rear seats since it really doesn't add much time to the whole process (just uses up more of the product).

If you clean & condition your leather with Lexol, you will literally be able to feel the "before & after" difference. The leather feels a bit softer and more comfortable and it really does look cleaner (even if you have black interior as I do, you do notice a difference). I have tried many different leather cleaning and conditioning products, and I find Lexol to be the absolute best. Zaino Z-9 and Z-10 do work very well, too. They are more durable (longer-lasting, stronger), but the leather always responds better to Lexol and it has a more natural smell. If you want to keep things really simple and cheap, you can pickup a bottle of Meguiar's leather cleaner & conditioner (both in one) and just use that stuff once a month. Cheap and easy and gets the job done, just not as thorough as Zaino or Lexol.

Ultimately, there is no "right" or "wrong" way. Some people swear by Lexol, some people swear by Zaino, and some people swear by doing nothing. At the very least, if you treat your vehicle's interior with respect and you take SOME kind of proper approach to maintaining it (particularly the leather), your vehicle will DEFINITELY take care of YOU later down the road.