Depends how well made they are. And given how most stuff these days is cheap crap, I'd wager ones installed today won't last that long as ones made in 1990.

The rubber seals often give out making them next to useless. Some of the ones on my house need a proper slam to close again. The panes can leak letting in water and getting internal condensation (although you can just pop those out and replace them without doing the whole frame).

Forever, if you don't ever intend on opening them and have good enough luck for people to miss when they throw rocks.