Help me answer the following questions

[ParisBashirian]

Despite having played for many years, I still have some uncertainties regarding the mechanics of item decay in the game. I am aware that items left on the ground decay when the player character is not present in that specific area, unless the items are stored in a vault or the Sher'Tul fortress. Due to this, I tend to avoid leaving zones that I haven't fully explored, as I am concerned about items decaying while I'm away. I have a couple of questions regarding this mechanic:

a) How much time does it take for items on the ground to decay after the player character leaves the zone?
b) Does the decay timer continue to progress even when the player character is in a different level of the same zone? If this is the case, does the decay occur at the same rate as when the player character is absent from the zone entirely?

[smithfield]

Artifacts never decay. (also RandArt?)
Mob drops do not decay. Time is not a factor.
Upon leaving a non-safe zone, any non-artifact that pc has dropped will decay
Safe zones are..
. she'rtul fortress
. locked chamber in the ruined dungeon
. the cave that branches off from the entrance to the astro-wotsits in EoR.