The supreme court opinion was essentially putting congress on notice not to try this again. They weren't really going to farther. If congress does it again, i'd expect them to be significantly more receptive to the argument that it is "unlimited copyright on the installment plan"

It has and will continue to be interpreted that way until a stronger defense against it is conceived.

1 million years would be pushing it, but there isn't a clear boundary. Eldred v. Ashcroft is the most authoritative case in U.S. law on this but it doesn't reach a clear conclusion. [1]

What's more interesting is the dissent in that case. According to Justice Breyer, the current copyright term is valued at 99.8% of a perpetual copyright (presumably because the present value of the future becomes increasingly discounted over time). Unfortunately, the rest of the court didn't go along with this as it would have implied throwing out plenty of prior copyright extensions as well.

[1] http://www.law.cornell.edu/supct/html/01-618.ZO.html

Then ... 1 Billion years!