Gas Tube & spare bolt are good to have.

Besides the bolt - spare extractors (with springs & inserts already installed) and spare gas rings.

Those seem to be the primary wear parts.

Secondary would be cam pin and firing pin retaining pin.

Lot of factors involved - ammo use, how you shoot (lots of mag dumps vs slow fire), environment you shoot in, your maintenace habbits, and the quality of your parts.

My most worn AR, a Bushmaster 14.5" M4 barrel, has just over 8k rounds on it since 1999 (I haven't shot it much in the last 2 years).  During that time I had to replace the gas rings once and the extractor/extractor spring once.

I also replaced a firing pin retaining pin because it was getting bent and hard to insert.

The gas tube was replaced twice.  The first time somewhere between 6K and 7K rounds (to fix a gas leak - it was leaking gas where the gas tube endered the FSB).   This fixed a random short stroke which had started occuring.  The second time was before I finished that case (I wanted to try a PRI Fat boy which I kept on the barrel - the change was not NEEDED, just something I wanted to try).

During this time I shot mostly PMC and South African M1A2 & A3, with a couple case of Q3131A, and a case of Wolf 55gr (the older laquered cases).

Based on what I have experienced I will replace my extractors at 5K rounds on my newer ARs (when they eventually get that much on them).  My gas rings are replaced when they fail the "bolt drop test" as outlined in the -23 on page 2-12 (test them everytime you clean your rifle).  The gas tube thing IMHO was a fluke - but I have spares as it IS a wear item (don't forget spare gas tube pins as well).

Quoted: .......how about Buffers?

I've not heard of a buffer problem, but a spare buffer spring isn't a bad idea at all (BTW there is a test in the -23 relating to measuring the springs to make sure they are in-spec).