In general, a long time… well over 20 years… but, it depends. For instance I had a pair of ESS AMD 1Ds… which had relatively inexpensive woofers with material around the woofers exterior that wore out after about 12 years… similarly my B&W subwoofers needed the cone suspension material replaced after 25 years. The smaller B&W 805 Matrix speakers (in my home theater) sound and are great after 25 years.
I think if you are talking about high end speakers (not ESS AMD 1D… only good, not audiphile) thirty years or more is not unreasonable. But, as in most things, it depends.