How long do VHS tapes' film take to become brittle and unreadable?

The main enemy of VHS tapes is overprint due to adjacent magnetic tape layers affecting each other. One of the first things you should do to maintain the quality of a VHS recording is to wind the tape from end to end so that the same magnetic areas don't overlap (due to the mechanics of a video recorder it's unlikely that exactly the same two piece of tape will overlap after a full wind/rewind). This should be done every 3-6 months. The rest is physical i.e. store them in a cool dry place out of direct sunlight.