Hi Martti,
I agree... wish we had this info for every job. There are ERQ's (enhancement requests) open currently asking to provide this. To answer your question though, if this job takes at least 25min to run everytime, then you be better off running it every 30min. Even if it overlaps a bit as long as this job is set as "incompatible" to itself, Clarity will never try and run another instance while one is already running.
--Kathy