Probably never.. I think they don't even want to do lifetime subscriptions (and they went away for a while years ago), but only do it because many people, including me, would _likely_ not buy a monthly subscription... and originally, for people to do comparisons with ReplayTV (which had the price built into the box even more directly than a Tivo + lifetime subscription).
Yes, I'm leaving wiggle room, because if the price went low enough, I would pay monthly... But in general, I'd rather pay one price, in full, _even_ if it in effect makes me pay more (e.g. I pay lifetime for a box and I get a new box in 4 years.. I'm losing the interest on that money if the monthly happened to end up the same price overall.) That's even leaving off the resale value of lifetime.