benignuman,
since you say its been a while, ill point you in the right direction. Rambam Hilhos Teshuva Perek 8 Halachah 8. Its clear as day that Olam Haba exists in the now and present, and that Olam Haze will never cease to exist. and if you have a rambam in front of you, notice the Raavad argues. The Rambam understands that Techias Hameisim and th coming of Moshiach are one time historical event that will cause people to finally believe in and serve Hashem. Other than that, everything in this world remains the same. Look in Hiclhos Teshuva Perek 9 Halachah 2 for more detail. I think it would be safe to say that the Rambam would reject any notion of an expiration date on Olam Haze.