It might be a consequence of unification theories. The ranges seem to vary between 10^31 and 10^36 years. See
http://en.wikipedia.org/wiki/Proton_decay
Recent experiments at the Super-Kamiokande water Cherenkov radiation detector in Japan give a lower limit of the proton half-life of 6.6×10^33 years.
Early grand unification theories, which were the first consistent theories to suggest proton decay postulated that the proton’s half-life would be at least 10^31 years.