Heatsink paste lasts ages, even when opened. Eeven the el cheapo stuff.
btw, in case anyone is wondering about actual use:
I meddle a lot in electronics, where a crazy overclocked cpu's heat output PALES IN COMPARISON to a mosfet in a power amp driven full load day after day (ie 200W of heat to be dissipated in a 1.2x1.2cm space, comapred to the 1.5x1.5 or the 4x4 heatspreader, whihc only has to dissipate 65-120W). Heatsink pastes simple lasts, it doesn't need changing, well meyba eonce every 7-10 years if used in extreme heat.