ToME: the Tales of Maj'Eyal

Even though I've been playing for years, I don't have a complete understanding of how item decay works. I know that items on the ground decay when you're not in that area, unless the items are in a vault or the Sher'Tul fortress. I tend to avoid leaving zones I haven't cleared because I worry about items on the ground decaying. Some questions:
1. How long does it take for items on the ground to decay after you leave the zone?
2. Does the decay timer continue while you're in a different level of the same zone? If so, does it decay at the same rate?