I just thought I'd give an anecdotal report of Vittoria Mastik One longevity. I wanted to see how my C50 would look with black sidewalls versus the tan sidewall Veloflex Criteriums that I glued to my Bora 35 wheels maybe 5 years ago. (I have other bikes, so these wheels don't get ridden constantly.) I removed the Criteriums last night. The glue was holding just as strong as the day it was applied and the tubulars were very difficult to remove. The glue is not brittle at all.
In case you are wondering, my gluing method is pretty standard: stretch a new tire for a day or more at high pressure on a spare rim; use a bit of emory cloth and rubbing alcohol to clean a new rim; use a brush to apply two "very" thin coats of Mastik on a new rim edge to edge and on the base tape, allowing all but the last coat to dry for 24 hours between coats; after the final coat, wait a few minutes, inflate the tire just enough to give it shape, and mount it while stretching the tire around the rim. Pump it up a bit more. If it is uneven, lift and center. Pump it to max pressure and push roll it on the ground. Deflate and use my thumbs and light pressure to confirm there are no gaps between the base tape and rim edge that are inadequately glued. If none, pump to max pressure again, confirm the alignment is still good and let it sit for 24 hours before riding. Having a beer or two during this process seems to help.
I will give the old rim a quick once-over to remove dirt or dust, but won't bother removing the old glue before mounting new tires unless there are loose/brittle or uneven chunks or thick glue buildup on the rim. Because I only used very thin coats originally, I won't see many big chunks or glue buildup. I will put two "very" thin coats on the new tire (24 hours between coats) and one very thin coat on the rim before mounting the new tire. The one coat on the rim will help activate the old glue and smooth uneven areas.
My process may be overkill and take longer than necessary -- I suspect I could use just one coat the first time rather than two -- but my anecdotal evidence is comforting that my method is effective. (I've seen the same with my other tubulars/wheels over the years). I nonetheless check occasionally how the glue is holding up by using the thumb test on a deflated tire.
Anyway, I realize I got a bit off topic, which was longevity of the glue.
“If you save your breath I feel a man like you can manage it. And if you don't manage it, you'll die. Only slowly, very slowly, old friend.”