I know I've gone as far as three maybe four generations of copying in the VHS format, and despite tons of garbage getting in each time they were still rather watchable (though using a video stabilizer during dubbing would have helped greatly).
I'd bet that with a good stabilizer during dubbing, tapes without bad sections (I often find one or more spots on a VHS tape that will play back with a noise streak even after the tape has been recorded over!
), some type of filter to prevent line noise from getting to the decks, a metal cage around the equip to prevent pickup of STRONG RF noise pickup, properly functioning VHS decks and a procedure of maticulously checking a copy for flaws (and rejecting flawed copies) before copying it again one could probably get a VHS tape to remain watchable out past 10 generations of copy (probably even farther for S-VHS-ET recorded VHS cassettes).