Depending on how often you turn on and turn off your gear, but tubes should last about 5,000 to 10,000 hours. For most people, they only need to be replaced every 2-3 years at most. But it's good to have at least a spare set around just in case one of them decides to die on you.
Most popular audio tubes are still been manufactured in countries like Russia and China. They are just not as good as tubes that were made during the golden age of tubes (1950s - 1960s) in most applications. The market for vacuum tubes are fairly small, comprises mostly of audio enthusiasts, musicians, and ham radio enthusiasts. But the supply is fairly steady. So don't worry, they won't run out. Unless, of course, your gear uses some obscure tubes that nobody is manufacturing these days. Even that, you will probably upgrade to something else before those tube supplies dry up.
And if you are that worried, buy several sets of the tubes now and save them for later use. The price of those tubes will only go up. (Better than investing in most stocks) I have "horded" enough NOS 6DJ8/6922/7308 tubes to last me several lifetimes.
And...you can always ask Doraemon to take you back to the 1950s with the time machine and get some good NOS tubes for dirt cheap.