I'm not talking about the years that a particular person or group can spend enjoying the hobby. Instead, I want to talk about the significance of years in game time.
When describing a game world's history, it's easy to get carried away and have changes occur so slowly that there's essentially a case of medieval stasis. The Great Collapsing Hrung Disaster happened in 300 AGC (After the Grand Conjunction); the Fruit of the House of Gusher was stolen in 350 AGC; and the Murloc Invasion began in 425 AGC, which brings us to the current campaign.
And in all that time, people are still using steel longswords, maille armor, and yew bows.
To be honest, I'm somewhat guilty of this myself, which is why I've worked on establishing a rough technology timeline for my setting. The "Old Era" (which would be when my OD&D campaign is set) has bronze as the main weapons technology, and the ballista is a relatively new invention. The "Middle Era" (where my AD&D2 campaign is currently set) has portable crossbows, though they are relatively rare. The "Late Era" (rules system to be determined) would have non-magical gunpowder just being invented.
It's always important to keep in mind, though, that sweeping changes can happen in under a decade.