How many years in the future is your story set?

The novel that I'm inching forward on (more like millimetering forward on) is set in the future, and that's got me thinking about what does and doesn't change in the future, or, to put it another way, how far in the future you'd have to go before something had disappeared or was forgotten entirely. Tech is easy to lose--it can be lost in 20 years, if it's replaced by other things. (This may not seem like a loss, but it is: the skill to use anything is still a skill, even if it's an obsolete s...