The discussion of auto draw has alternated from active discussion to oblivion. Somewhat recently it's found root again with thoughtful people.
It used to be that 200 moves (100 per player) was the consensus but nothing was ever tried with that back in the beta days. Now, the 100 moves (50 per player) number makes a lot more sense to me for just the sheer time factor. 100 moves times 15 seconds is 1500 seconds that someone could still waste and that's 25 minutes. Right now we would punish a draw refuser after ten minutes, so 100 moves for an auto draw per person doesn't make much sense with a bad time waster.
But then, 50 is about 12 minutes at the worst and is still poorer than our current ten minute rule, except that no screenshots and reporting are involved in a 50 move auto draw and that's worth a lot to everyone, no doubt.
The worst thing with auto draw is the number of repetitions that the rule would allow. If you say the 50 move counter gets reset every attack, then a bad time waster might easily kill one lowly piece every 49 moves and cause more frustration than before.
So, somehow, there would have to be added rules to make the auto draw more sophisticated than just the simple 50 move line. Some of the ideas have been discussed this year by TheOptician and The Prof and others but I can't locate that discussion at the moment.
EDIT: I found the discussion I was thinking about here: