It really depends on the motherboard and the quality of the capacitors. Some motherboards have solid rubberized sides to their capacitors, which helps keep them vibration-free, as well as gives them a bit more protection from moisture. These tend to last around 3-5 years before needing to be replaced. Other motherboards use cheaper, lower-quality capacitors that are filled with a pressurized liquid instead of solid rubber. These capacitors are generally only rated for 2-3 years before they start drying out and losing their effectiveness.