In the case of the Grand Floridian, it's pretty easy to make an educated guess on how long some of them have been burned out because there are SO many that aren't working at all. It would be more difficult to guess if it was only one or two within a row of 50-100 bulbs. While they may or may not have fixed some in the recent refurbishments the resort has undergone, as of my most recent early 2012 visit the burnt out bulbs were in large clusters. We're not talking just one or two burnt out bulbs within a hundred or so, more like a very large fraction within a count of a hundred. That is horrible show.
I can forgive a small quantity burning out provided they keep on replacing them. Defects happen and I get that. But in the modern era they keep them that way for a very long time until a major refurb happens. It makes one wish they'd convert them all to LED's if they won't take the personal initiative. Those can die as well eventually (or suffer from defects), but at least it's more difficult for them to do so. They have made a switch to LED's in some cases apparently (Mad Tea Party apparently uses LED's for its popcorn lights now I think), though I don't know whether Main Street or Grand Floridian changed to LED's in their recent refurbishments.