Having learned recently of quite a few double glazed windows and roof panels getting "misted up"/"showing condensation between glass layers" after 5-10 years I am asking myself what the life-expectancy (say the average and 10% loss rate) actually is. Trying to talk to one or two manufacturers suggested that this may be a delicate issue for the trade. What is the experience generally on this topic? I would hate to install expensive and well made windows only to be repairing them every 10-12 years! Nor very green for one thing. Maybe secondary windows with good seals and frame insulation done internally has a place along with moveable insulated shutters etc

10 years is the normal guarantee in plastic or fully-drained wooden frames. the edge seal fails and will let in water vapour eventually causing condensation.
i make frames expecting them to have the units changed in their lifetime.
it will generate a lot of waste, but i suppose the glass can be easily recycled once separated from the spacer. i wonder if the council/waste contractor do this though?
for your typical house with smallish windows i think there is definitely a good case for the idea that good secondary glazing, curtains, shutters, proper seals on openers etc; as being greener even if the U value is worse.

you can cut through the spacer with a biscuit jointer but you will nacker the blade. i dont think its really worth it.

You can split units easily enough on a diy basis - it's only gunk after all - there's no mystique involved, just time. Glaziers aren't likely to bother at the present time, it's quicker for them to cut new glass. But what if you dismantle a unit? Can you diy its re-manufacture? Surely possible if you can source the materials. But you may have to lodge something in the vacant window whilst you're at it.

In other words, the INDUSTRY is not currently motivated to recycle, but individuals may be.

i was told to avoid dark coloured spacer bars as they expand and break the seal over time?

Is there any substantial difference in the manufacture of large units for commercial buildings? Having units fail high up in a commercial building would be very expensive.
I have often wondered about the long term costs of some projects featured in 'Grand Designs' which use very large glazed units. The number of manufacturers for such units seems to be very limited and delivery times are often a problem. On a more modest domestic scale how can one check that the d/g or t/g units are being made to a high standard? Possibly this would be more relevant with UK manufacturers rather than established major manufacturers in Scandinavia.

I am not at all sure that "the exotic gas filling" really can stay between the panes for very long. Movement out in exchange for air must be going on all the time.

It's quite a tricky question because the technology has changed over the years. The materials and methods used 20 years ago may not be the same as those in a unit bought today.

I had some fail after about a year. They lasted just a little longer than the firm that made them! I've others that have lasted 20 years.

So far I've not had a complaint from any customer that I've supplied with the type of units I've been using for the last six years.

SteamyTea, that is a v.helpful article. I wonder how many householders or even installers know these things never mind know how to apply the principles in their "refurbishments" of windows. All the other comments very helpful too. My reason for doubting the longevity of filling with argon or what ever is that gases find their way through materials and interfaces without the need for "holes" in the ordinary sense, just as pentane finds its way out of insulation. My guess is that exchange with the gases in the air is going on long before the "condensation" issue raises its ugly head.