Not sure five years is the number to fixate on as humidity, temp and other weather factors will determine the mileage you will get out of even the highest quality sealer. Soil contact has already been mentioned but sprinkler overspray can speed wear too. Stay away from icky wax based sealer products like Thompson's Water Seal.
Well maintained cedar heartwood, with treated soil end posts, can last a long time though and choosing long lived fasteners can really lower your maintenance. I did minor repairs to an antique cedar lattice work fence around a property a number of years ago now and am quite proud of it today. I put two tones of a high end solid stain on it. It was close to 100 years old when I got to it and many fasteners had had it but the wood could, conceivably, be around for another 100. Gates had large round openings in them and the kids climbing through them had done damage to that lattice work. The rest was fine.
I have worked on antique cedar and cypress heartwood house siding that is over a century old. The weak link has usually been corroded fasteners or layers upon layers of paint.
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Last edited by sdsester; 10-17-2012 at 05:03 PM.