(*) Infinite, that is, until the Chinese and other foreign nations stop purchasing our debt.
Going forward, in general, the Federal Government will not be able to rely on the Social Security Trust fund purchasing the bonds it issues. It can cut spending, raise taxes, or find other buyers who want to purchase bonds.
There is no particular reason to be concerned about the ability of the Social Security system, for years to come, to support it's obligations to the workers who have paid into the system. Unless, of course, you fear that wealthy elites will manufacture a bogus social security "crisis" to dispossess the savings of workers and continue to enjoy (federal spending/tax cuts) that can not otherwise be sustained.