hey mate
i myself am unsure of a difinitive "rule of thumb" answer to your question, but i honestly believe it really depends on the set ups and conditions.
I would imagine that how it was sealed inside (pond armour/glassed and expoy) could be a contributing factor in the life of the unit. However probably the most important and possibly over-looked factor would be the humidity of the area in which the tank is housed, if uncontrolled im sure in some areas and some climates the wood would very likely eventually rot. I mean a 1000+ gallon monster housed in some yanks cement basement with poor ventilations gunna have moisture issues if the evap isnt considered and therefore more thank likely suffer wood rot eventually (not to mention the rest of the house). As aposed to the same set up in the same climate with the humidity issues addressed and taken care of or the wood properly treated to endure the conditions.
Under the right conditions the ply or whatever other wood is there would absorb moisture and then overtime you would loose structural integertity then.............

BOOM. In smaller ply tanks i dont really see this being a major issue, But as this is MFK im basing my opinions on 1000+ gallon setups where the evap'd water is in the air in a confined area (house, basement, Garage etc).
I would imagine a Quality built plywood MONSTER which has been carefully thoughout and planned could easily last well beyond a lifetime.