How often do pens need to be reconditioned?
It is difficult to estimate how long newly-replaced cork and rubber seals and rubber ink sacs will last. Certain inks are harder on sacs than others, and there are also significant variables such as temperature, humidity, and levels of common pollutants such as ozone. Nonetheless, it is worth noting that one still finds many Esterbrook lever-fillers from the 1950s and 1960s with usable sacs; the same is the case for Sheaffer Snorkels of the same vintage.
Judging from empirical evidence, it would seem that rubber sacs should be good for several years at least and may well last for much longer. Cork or synthetic rubber seals, whether in piston or pump fillers, or in safety pens, should last at least as long, and the special vinyl sacs used on Parker Aerometric fillers would appear to have an average lifespan in excess of 50 years.
UPDATE: In the last several years, we have seen a dramatic upsurge in premature sac failures. In nearly all cases, the problem appears to be new ink formulations, whether offered by old-line pen companies no longer concerned with compatibility with their older products, or by boutique ink makers striving to offer novel colors and qualities without heeding their long-term impact on the pens in which they are used. Read more about vintage-appropriate ink selection here.