There are a number of ways to do this. One way that is easy (and loose) would be to use the following tags.
SP_IS1D0_SD360_PCR
You'll need to add the group info. I forgot how to do that without my toolset and notes here with me.
The NESS tags I gave you mean the following:
it is a NESS spawn point, initially active, has 0 initial delay to a spawn, a 360 minute delay to subsequent spawns, only spawns when a PC is in the area, and once the PC leaves the whole thing refreshes to initial conditions. Refreshing to initial conditions means that the initial delay of 0 minutes will be adhered to rather than the 360 minute.
I chose 360 as an arbitrary delay. You could put any ridiculously large number there.
There are more precise ways to get the result you want via scripting, but I figure that if you knew how to do that you wouldn't be posting the question. Its a bit more complicated and involves scripting.