Length viability depends on many factors and can vary from genus to genus or species to species. It depends on how the seeds were stored etc. One study on Dionaea seed germination and viability found that seeds of this species lost all viability if dry stored at room temperature after 45 days. Where as fresh seeds (just ripened) sown on moist media would begin germinating after 45 days. Seeds dry stored and refrigerated can remain viable for many years, perhaps even indefinitely. Bob Ziemer will relate to you how he germinated 20 year old seed. And then there are those wheat and other grains that have been germinated from Ancient Egyptian tombs.
Most CP seeds have reputations for short viability (several months at best, a few weeks for Dionaea and some Drosera) periods if not stored properly.