I saw some recent discussion about the length of time it takes to cap divination tools, and whether or not Destiny Cipher is worthwhile. Here's the data I have.

All tests were done using this divination prism : https://elanthipedia.play.net/Item:Bloodmist_garnet_prism_ensconced_in_a_black_gold_spiral

You turn your prism about, carefully evaluating the craftsmanship.

The refraction of the prism is comparable to quartz.
The prism is very well-balanced.
The interior has several major inclusions.
The planes of the prism are well-cut.
The prism is rather new.
It appears to have seen heavy use.
The overall craftsmanship is spectacular and the prism should prove extremely resistant to damage.

95+% of predictions were done on full pools with Destiny Cipher up.

From Sept 12 2018 to October 3 2018

261 predictions performed
18 cracks = 6.9% (decrease in a bond quality)
66 serenity = 25.29% (increase in a bond quality)

Initial state (on Sept 12)

logs/main/main.log.2018-09-12:[12:12] Two vertices are bound to probability.
logs/main/main.log.2018-09-12-[12:12] The prism is listless.
logs/main/main.log.2018-09-12-[12:12] It has a nearly random prophetic composition.
logs/main/main.log.2018-09-12-[12:12] It is almost certain to cause unintentional internal inversion.

Current state (on October 3)

logs/main/main.log:[12:46] Four vertices are bound to probability.
logs/main/main.log-[12:46] The prism thrums with life, agitating the very air about it.
logs/main/main.log-[12:46] It has an ordered prophetic composition.
logs/main/main.log-[12:46] It is very unlikely to cause unintentional internal inversion.

So it seems to be about 50% of the way toward being capped after 20-ish days (duration seems to be capped, others seem to be around center point)

I had an average-craftsmanship prism before this. I didn't keep logs but, anecdotally, it seemed to have much slower improvement.