How long should a barleywine be aged before bottling?

I just drank a bottle of Barley Wine that I bottled on July 2, 1993! It still had carbonation; no bad flavors; grains forward, hops subdued. I've lost all the records of what and how it was brewed, how long it sat in primary and secondary etc., but I wish I had some more hiding in the back of the fridge.

There are advantages/disadvantages to each. Aging in bulk will minimize the oxidation that will occur (all other things being equal) as it has less surface area. Some level of oxidation can be considered a benefit in a barleywine, but it can go too far.

So I think if you are looking at keeping a barleywine for a very long time, it is best to do the primary aging in the fermenter, assuming it is well sealed (air lock) or in a keg (which also offers you the easy ability to sample it at different ages). Bottles are an advantage as it will free up a fermenter and potentially make storage easier (as well as make sample it at different ages easy, also).

A couple other things to consider, you're going to lose some carbonation on bottles, especially if you keep them for a long time. Corking can help this, but again, bottling after the primary aging works well, also. And a keg or a carboy is going to have more thermal inertia than individual bottles so will be less likely to suffer from temperature changes.

They will actually age just as well in the bottle as they do in the fermenter, assuming they are kept in the same temperature controlled environment. In fact they will be better off when it comes to UV damage since they're in brown glass, not clear.

You are right that many times big beers are fermented for longer periods of time, but that's because as the alcohol content rises and the oxygen is depleted the yeast slow down. You wanna make sure that you've fermented all of the sugar out before bottling. I currently have a Belgian Tripel that was brewed over a month ago and is still fermenting (the predicted abv is ~9.5).

Typically speaking it's best to age in the bottle, not out.

Bottle once fermentation is complete and the beer has cleared out, same as always. However you want to let it stay in the bottle much longer than your standard brew before consumption; around 6 months is standard. Reason for that is because there are hot alcohols and (if you're brewing american style barleywine) big hop flavors that will take longer to blend.