lovely question! I guess that, since no ferro boats are built on a standard "production line" basis, there can be no standard life span. depends how they're built, used and looked after. above all, it depends on their worth in the market, since almost any boat can be renovated. it depends on the market value whether it is worth doing so .
an even more interesting question (because there are far more of them about) is the lifespan of grp boats. at the moment, despite numbers of new boats coming on to the market, old boat prices arent going down. its as if your old car was worth what you paid for it when new. personally, I cant see this continuing, and if the market value of the old boat drops, so will its effective lifespan. but whats going to happen to them all. you cant leave them on the salt flats to rot away like old wooden boats. and the impoverished final owner wont want to pay to have all that old grp disposed of in line with the waste disposal regs.
we are beginning to see this problem at my club, where we have 50 or so mainly old and unloved boats on the hard all year round including some very tired ones that no one wants
some of the ferro barges built for D day are still afloat! Like all things it depends on the build quality. Ferro gets stronger over the years, but internal stresses can fracture it (see concrete spalling on buildings) pure ferro seems to last much better - a lot of bridges built of ferro before they were re-inforced with metal are still perfectly good.
Actually ferro, means ferrus (Iron) re-inforced concrete. Ferro boats have an armature of iron bar and iron mesh, which is then plastered with pure cement, fine sand mixture.
some idiot at our club set fire to a grp dinghy last year - the smoke was unbelievable, as was the speed with which it burnt. but the worst problem was dealing with all the glass fibre that was left behind.
The concrete component has an almost indefinite life. A colleague took core samples from some of the Thames wartime fortifications about 10 years ago and their properties were at least as good as they were when built. There was some speculation that the strength had actually increased with life.
Properties of the steel reinforcing are completely different. If water gets inside, then the steel corrodes, expands and bursts the concrete. Plenty of 1960s bridges and buildings will testify to this. So a professionally built boat that has been well looked after should last for an almost indefinite period. Badly built ones may already be dead.