There is no good way to tell when a repeating group in an automation is finished from within the repeating group. This is in part because there is no guarantee that items in the list will be processed sequentially. Items in the list could be processed simultaneously or out of order.
This you will need to use something outside of the automation to determine when the repeating group is over. This will depend on how your list is generated. For example, if your automation processes linked child records, you could have the automation update a field that says the child record is done, and then have a rollup on the parent record check if all records are done. Finally, have a second automation that triggers off the rollup to do whatever additional processing that you need.
