• 25 years is the nominal operating life of Japanese submarines, to keep the yards busy
• 30 years is the nominal operating life of American attack submarines
• 40 years is the nominal operating life of American FBM submarines

The time spent in reserve probably should not count against total life expectancy.
The units K-239 and K-276 were initially in service for about a decade, and
then spent one and two decades, respectively, laid up in reserve.
In principle, these two boat, with incorruptable titanium hulls, might be
expected to remain in service until the mid-21st Century

A plausible fleet management ideology would see the pair of Barrakuda boats
return to service in the mid-2020s to replace the pair of Kondor boats, which
would by then have been in service for some three decades