Eric is correct, the manifolds should last a very long time, but they have one area that can corrode away and require them to be replaced. The water jacket is on the top and front side of the manifold. The back side and bottom are not jacketed. While not in contact with raw water, this unjacketed area at the exhaust end of the manifold can thin from corrosion (inside and/or outside) until there is not enough surface left at the joint to seal against the tailpiece gasket. Also, the boss inside the manifold that contains the threads for the tailpiece bolt in this dry area can corrode away until the threads won't hold. The unjacketed sides of the tailpiece can also thin in the same way.
The 6V53s in our 1965 41DCMY were installed by the PO during a repower. Based on the serial numbers they are much older than the boat, and I wouldn't bet they had not been submerged at some point prior to installation in this boat. I replaced all 4 manifolds and tailpieces in about 2003 due to the corrosion described above. At that time I was able to buy new manifolds and tailpieces for the 6V53 from Detroit Diesel. I don't know if that is the case for the 8V53.
The thinning only seems to occur at the exit end of the manifold. The manifolds are reversible if the tailpiece and thermostat housing are removed. If you inspect for thinning and catch it before it goes too far (as mine did) you could reverse the manifolds and double their useful life.
If you don't want to remove the tailpiece to inspect each manifold for thinning you could measure the dry area's wall thickness using an ultrasonic thickness gauge. You only need to worry about the flat sides (back and bottom) at the exhaust end between the tailpiece joint and aft exhaust port. Also check the tailpiece itself at the bottom and backside.
