How long should a frame last?

Titanium lasts if it has been welded properly. If not, the welds can crack.

My nephew is still riding a 1990 Look 196 carbon monocoque bike. Solid as the day it was (over) built.

Would be interesting to revisit this in 5 to 10 years, when people are reporting back on longevity of carbon frames. Recent price drops and technological advances are making them a mainstream choice, so I guess you'll be hearing about failures pretty soon if they're going to happen

As above... depends on what you have done to the frame.

I've busted a few aluminium frames over the years and apart from one (catastrophic front end failure), they've always just displayed an obvious crack when being cleansed. No drama.

I've had more dramatic failures in steel frames back in the day.

The real answer is: it depends on the frame design, age, its material and its treatment.

Generally, heavier frames will last longer than lighter ones. They're sturdier and survive crashes and falls better.

Older frames tend to last longer than newer ones (this sounds Irish, I admit). Modern frames are lighter andstiffer. But this means using oversized tubes that have much thinner walls, which makes them much more prone to failure in a fall. Older carbon frames were overbuilt (see heavier above), so a 20 year old Kestrel SCi 200 could still be ridden, while a 2014 Kestrel Talon will probably not last nearly as well.

Titanium will, all things being equal, last longer than anything else. Steel will last for a long time but is prone to corrosion if not treated. Aluminium can last for quite some time, but it is a fundamentally flexible material to use for cycling, which is why it's extruded into oversize, very thin tubes (see above). Carbon will eventually become flexible and fail, not because of the carboin itself but because the torque stresses the resin. Eventually the stress cycles cause the resin to crack which causes the flexibility.

Finally, you have treatment. The more watts going into a frame and weight going on it, the more it's stressed per cycle and the sooner it will fail. Ditto for maintenance. So a large, powerful rider that neglects their kit will be buying frames a lot more often than a lighter rider that looks after their bike.

Rules of thumb; titanium and steel will last decades if properly looked after (particularly for steel). Alloy bikes should last a similar time, but a very light alloy bike is probably built for a fast time, not a long time.

Modern carbon bikes are built very light; maintenance is less an issue than usage. Ride a good carbon bike religously (like a pro), at their intensity, and you will get a couple years out of the frame before it starts to flex noticeably. At this point you should bin it (or relegate to a training bike) if you're a racer as the frame is absorbing precious power with every flex.

Then again, how many of us are capable of putting out 400+ watts on a sustained climb? Chances are that a well looked after carbon bike on a non-racer will last just as long as an alloy bike.

My daily ride was built in Glasgow in 1953, from Accles and Pollock KrOmO air hardening tubing.

It's had a few re-enamels, but I must admit in 2012 there was no longer enough metal in the drop out to support another thread being cut and I had to use a frame saver......

My riding buddy commutes daily on a 1970s steel machine, made in Italy by Ciocc but badged under the name of a local builder. It copes brilliantly with all sorts of weather and very rarely gives trouble.
He obviously loves the thing to bits, keeping it immaculately dressed in Super Record and he reckons it's definitely more comfortable than his 21st century carbon "best bike".

As a raw material, and as an alloy Aluminium naturally age hardens. When you buy a brand new Aluminium frame it's almost certainly been artificially aged / hardened. As the metal ages it becomes harder and potentially more brittle until it reaches it's point of maximum hardness at which point it can then begin to soften.

How many years you can expect to get out of a frame depends on a number of factors. If you wants frame for life, your best bet is Titanium or Steel (provided you don't let it corrode).

You're right about the breaking point too. Ti and steel normally give you a warning when their about to break, normally a tell tale creak or flex. Ali and Carbon on the other hand can just go!

Off topic, but anyhow... My dad inherited a reynolds steel "fast tourer" while at university in Cambridge in the 1960s. It was an Elswick Lincoln Imp. All four of his children were carted around in a lethal non padded child seat on the back. He rode it to work nearly every day for at least 25 years, carting several kilos of exam papers in panniers up hills in Exeter.

My brother and I eventually persuaded him to trade it in citing the fact that the seat pin was rusted in and there was a crack around the seat tube near the bottom bracket. God knows how long he rode it like that for.

Anyhow, that machine did more than 8 years.

I've an Italian steel framed bike that was made in the early 1970's and I see plenty of old steel bikes around.

Not worried about my alu framed Trek as they give a lifetime guarantee.

Yeah. Just don't put them in the dishwasher.

I've broken a number of bikes, an alu mountain bike eventually sheared at base of BB, after about 5 years, salt corrosion I think, then three light-ish alu road bikes in fairly quick succession. Random shearing at different points each time - seat tube, BB and chainstays..

All covered under warranty.

I now use a titanium frame (with decent warranty) and touch wood...

So IME lifespan of alu less than the 8 years quoted by OP

I'm not sure what the expected lifespan is, as I've never known a cyclist to keep the same bike as their primary ride for 8 years, as something newer and shinier inevitably comes along!

I have seen some aluminium frames come into my LBS from various manufacturers crack around the BB and the point where the seat stays meet the top tube/seatpost, I imagine these areas are probably some of the most highly stressed parts of the frame. Aluminium breaks are also rarely repairable unfortunately.