Summoning dolls can stick around indefinetely, as long as they do not move too far from where they are summoned (they will turn red when nearing that boundary). Entering or exiting buildings, caves, the battlefields, etc will kill them, as will using the teleporter pyramids over long distances.
I often left the skeleton at the start of an area to load loot into, or sell it if there were merchants.
Then something is definitely wrong, because after summoning my skeleton stands in the same spot for 30 seconds, then disappears. Poof. I've resummoned him time and time again, and the same thing happens. Damn. This really is irksome, since the use of summoning dolls was such a big selling point of BD.
Thanks for the response, though. I figured it was some kind of bug in my particular game, but was hoping there was a cure somewhere. <img src="/ubbthreads/images/graemlins/disagree.gif" alt="" />