I learnt these from the old thief forums, what I remember off the top of my head regarding the mechanics of stealable artifacts:
Respawn rate of each level of stealable is double that of the previous level:
ie
Level 1 = 1 hr
2 = 2 hrs
3 = 4 hrs
4 = 8 hrs
5 = 16 hrs
6 = 32 hrs
7 = 64 hrs
8 = 128 hrs
9 = 256 hrs
10 = 512 hrs
11 = 1024 hrs
12 = 2048 hrs (85 days, about 3 months for ruined paintings)
The actual respawn timing is randomized by up to +/- 50% of the respawn rate. So the respawn timing of the lvl 12 ruined painting ranges from 1024 to 3072 hrs (approx anywhere from 1.4 to 4.2 months).
If someone really need that painting bad, well, they can try using a script 24/7 to camp for 3 months. But I suspect they would run a gold farming script for 3 months and make enough gold to possibly buy 10 ruined paintings.
Every 15 mins, the server does a check to see if each arty is ready/should be respawned. All arties that it decides to respawn at that 15 min interval will be respawned simultaneously in all 3 dungeons (Doom, Fan dancer, Tokuno mines). So if you see an arty spawning right in front of you, it's a good bet to check other respawn spots.
I just try to make my daily rounds once a day (actually, monthly rounds seem more accurate). So far I have scored both Buddha statues, 1 of the fish sculptures, all the different sword stands and the flower in the succubus room. Plus tons of the other stealables.
1 thing about the Zyronic Claw, can anyone confirm where it spawns now? I remember the spawn point was moved a long time ago, but not sure where, would appreciate if someone can point me in the right place. Thanks!