If you are using a transaction group, you could create a DateTime DB tag, then add it as a read-only item to the group (read-only in this case only means it doesn’t get inserted into the database; they can be written to). Next, add a DateTime expression item (triggered if the group itself is triggered) with an expression of “now()” that writes to the read-only db tag item (under the expression items general properties, under Target Type, select other tag). The tag will then always hold the DateTime of the last group run. You can check that and alarm off of it using a DateDiff expression (might need another tag, I’m not sure off the top of my head if you can alarm on an expression itself).
Hope this helps,