I think it might be more realistic to pick a number, say, "100 A-years". Meaning still sound color using Ardenberg methodology 100 years from test. Ink and ink/paper combinations could then be defined as having a 100 A years rating. Above 100 years is just fanciful, even less probably.
Quite correct. The "A-years rating" you have in mind is what Aardenburg actually publishes when it publishes its Conservation Display ratings. Only the rating is called "Megalux hours" not A-years. Actual years to invoke noticeable fading depend linearly on the illumination level in the location where a print is displayed. So, a print rated by Wilhelm Imaging as lasting 100 years at 450 lux for 12 hours per day average illumination takes 200 megalux hours of light exposure to reach the WIR criterion for allowable fade, and it can even last 200 years at WIR's assumed faded print quality expectation when you lower the average 12 hours daily illumination level assumption to 225 lux, or 400 years if you lower the assumed light level to 112 lux, or only 50 years if you raise the assumed light level to 900 lux, or 25 years if you choose 1800 lux, etc. etc. That said, and as noted in these examples, Megalux hours precisely translates to a corresponding "years on display" rating when you choose 225 lux as your 12 hour per day illumination goal (very easy to achieve in practice). Or, as one more relatively extreme yet real world example of illumination dependence on time to reach specified fade level, the print can only last 10 years if full sun routinely hits that same print at a glancing angle through a window thus raising the daily 12 hour intensity to 4500 average lux (10x the WIR assumed fade rate). But the 200 megalux hour exposure dose needed to cause the predicted amount of fade remains the same in each and every one of these "predicted years of life" extrapolations.
Hence, "years on display" predictions using only one underlying and often overlooked assumption of illumination levels are grossly misleading, whereas referring directly to an allowable megalux hour exposure dose removes this very real world illumination variability from the published rating. And for the record, these figures I have provided as examples here are not just hypothetical. They represent real world illumination conditions where photographs and other works of art often get displayed on a routine basis when located in non museum environments. Even most museums don't always adhere to a one-size-fits-all illumination level for museum lighting.
As for 100 years of actual "print life" on display being a fanciful expectation, I beg to disagree. Again, 100 years is achieved by a print process with a 100 megalux hour light exposure rating if you assume 225 years or less average daily illumination for 12 hours per day. And it's not not hard to keep images nicely illuminated on display at 225 average lux intensity or less over a 12 hour daily period. Moreover, there are indeed numerous printer, ink, media combinations I have tested which exceed the 100 megalux hour rating. Then again, there are many which fall far short.
Likewise, I have many traditional photographs handed down in my family that are near or even past the 100 year lifetime mark. Many of them were nicely framed at the outset and kept essentially on continuous display, albeit most likely well under that 225 lux/12 hour per day lighting level. Some look like they were made yesterday! Others show some patina of age, but still in very good condition. The badly faded ones are problematic at best. I really admire the photos in great condition! They are amazing to look at without having to resort to costly and time-consuming digital or physical restoration methods, and they capture not only a bygone era but the style and interpretation of photography and photographic printing from that earlier era as well.
cheers,
Mark
http://www.aardenburg-imaging.com