In short, it depends on the dock…
Just as with a house, the duration of a dock should be factored upon who engineered the structure and the quality with which it was built (e.g. materials + construction).
Also of major concern can be nature itself — sun, saltwater, storms and/or marine organisms. All take their toll over time, impacting the structure’s safe lifespan.
A suggested replacement cycle may be at 25 years, plus or minus. This is a rule of thumb, one that’s highly subject to the quality of engineering, construction and the adverse impact of nature.
Before ever moving forward with a property that has a dock or building it anew (@ area av. $110/board ft, as of 04/2013), I definitely recommend having the entire structure thoroughly inspected by a knowledgeable marine contractor.
As your Realtor, please know that I have time-proven, licensed professionals to help and advise you along the way. (And no kickbacks, not ever!)
Tammy Thrift, Realtor
Serving Your Real Estate Needs, From Colonial Williamsburg To The Chesapeake Bay!
Actively licensed with Long & Foster in the Commonwealth of Virginia (USA)