Oldbittercraig,
I'd suggest at least a week or two, but I'm not sure of any set schedule. However, it seems that the longer you leave it, the more 'interesting' the encounter. In my last game I left it to just before the party headed off to spellhold.
Kez
P.S. (spoiler)
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in that particular game the encounter was a bone golem. In the current game I only waited about a week and found a mere rune assassin.
