10+ years is a conservative estimate but there are factors to consider. Some people collect vintage/antique shaving brushes that are still usable today. The main factor being how much use they had in their lifetime.
1. Materials Used
2. The Construction:
A key point. How well was the shaving brush made initially? The knot is glued into the handle, this glue can deteriorate overtime but it is easy to re-glue or replace the knot if this happens. If the handle is made from more than one material this may also present a point at which it can break if dropped or mistreated.
So what else can end a shaving brushes life?
Dropping it and breaking the handle is the number one culprit. Leaving it soaking in extremely hot water is not recommended as this can damage the bond of the glue holding the knot in. Other than that you are likely to get a long long lifespan from your shaving brush.