This is not an easy question. It’s similar to asking “How long does a car engine last?”
Answer? “It depends how you drive it!”
The rate at which a valve wears out is determined by how many hours it has been on and how hard (loud) it has been played. There is also a mechanical component; if the amp has been bashed around a lot, the valves will not last as long.
Still, everyone wants a figure and so 500-1,000 playing hours is a good ballpark number.
Like car engines, valves don’t always break down completely, they just gradually get worse. So if your amp is pushing 1,000 hours and it doesn’t sound like it used to, chances are that new valves will make all the difference.