I am a quantum physicist...
...kind of...
...OK, used to be, but still...
There is no one answer here. How much fading depends on the amount of light present and over what period of time. That's the obvious part. The less obvious part is that different colors are thanks to different molecules. Different molecules will fade at different rates and so guitars with different dyes will also fade at different rates. Some may not fade appreciably at all, and as we all know, some have faded almost completely. Even two different dyes of a similar color can fade at very different rates. For some colors, it's not just UV that will cause fading but even blue light could contribute to it too. (The guitars that have faded appreciably did not spend their entire life outdoors in bright sunshine, right?)
My question is has whatever fading process that was going to happen already taken place after 25 yrs or so?
If a particular color didn't fade in the last 25 years under normal lighting conditions it probably won't now. However, 25 years in the darkness of a case almost certainly does
not protect a guitar from future fading.
So what to do? Two choices...
1) Case queen everything. It's the only way to avoid any fading at all.
2) If you are willing to accept that some fading will occur in some guitars and are still happy to display them you can minimize fading somewhat by drawing the curtains when not using the room. If you can identify which color guitars will fade the fastest, place those furthest away from the windows.
Assuming you switch off lights when you leave the room I don't think the indoor lighting is as much of a concern as daylight because of the reduced exposure time and intensity compared to daylight. To err on the safe side I would favor warm lighting (i.e. low color temperature) with a high CRI, such as the good old-fashioned incandescent bulbs, tungsten halogen bulbs and some modern LEDs, and avoid fluorescent sources, (e.g. long fluorescent tubes or CFLs, because they do produce UV light as part of how they work).
Of course even that won't prevent fading, but slow it down somewhat.
(A huge part of PhD was Organic photophysics, I was specifically interested in how carbon-based dye molecules absorb and emit light. Thanks for the trip down memory lane!)
