For those who wanted the answer to the title in these comments, paraphrased it was: with proper maintenance, indefinitely. The "with proper maintenance" phrase should be pointed out to management next time they think something we build should last forever; even these remarkable structures that appear formidably unchanging undergo continuous, hands-on, expert maintenance.

There is a tangentially-related Straight Dope thread [1] asking "Is the Hoover Dam concrete still curing?", because if you take the tour (highly recommended for hackers with an affinity for industrial- and megastructure-scale engineering), you will be told "the concrete is still curing!" by the guides.

This is apparently not technically precise; someone from the concrete industry please correct me if I'm wrong here. I gathered from a civil engineering page that Hoover Dam's concrete is considered cured, but it is still undergoing the hydration process as long as water is present in the concrete. [2] I think the Hoover Dam guides can be cut some slack for colloquially using the terminology "curing" in light of this technical distinction that most laypeople won't understand.

[1] http://boards.straightdope.com/sdmb/showthread.php?t=258217

[2] http://www.engr.psu.edu/ce/courses/ce584/concrete/library/co...