spark plug replacement

How often should spark plugs be replaced in a 96 civic?

as long as the spark plug is working properly, the gap is right and the electrode is not round up (you can service the electode by filing it and re-gap it) then you should be fine (except for carburetor cars and those are not computer control).

I think the OEM manuel says somthing like for every 100,000 miles, the spark plugs should be replace.

The manual actually says that you should check your plugs and replace if necessary, every 15,000 miles.

Always good to check your plugs often and replace them if they needed. If you forget to check though, when they've gone bad, you should notice a distinctive stuttering. When this first happened to me, I described it as feeling like the engine couldn't get enough fuel. It was actually bad plugs not firing properly.