According to Nikolay Solovtsov, the commander of the Strategic Rocket Forces, the older modification of the SS-18 missile, R-36MUTTH, will stay in service until 2007-2009. Given that these missiles were deployed in 1979-1983, their expected service life is being extended to about 26 years. Solovtsov said that the newer modification of the missile, R-36M2, will be around until 2014-2016. This also means a service life of about 26 years - R-36M2 missiles were deployed in 1988-1992.
Solovtsov also said something to the effect that Russia can develop a new heavy missile to replace SS-18, if it needs to (which some wrongly interpreted as if there is a plan to do so already). Well, I guess it could, but does it really need to?