Asking how long a cpu will last is problematic at best. 3-5 years for any system component (in terms of will it run XXX or YYY) is sheer guesswork. Physically, if you do not abuse the cpu (massive o/c) and keep it properly cooled, it should easily "last" 5 years. Chances are it will still play most games, but at differing levels, etc depending on what is required of a cpu/gpu in 3-5 years.
The 4670k will be fine for a little editing/recording as long as you are not expecting to be a video production machine.