One approach I've seen is to collect items in a list and then process the list when you flush the batch when it hits the batch size but also setup an executor to flush the list on a periodic basis.

I'm wondering if the list is needed or whether you you simply just need to flush the entity manager periodically.

Do you speak of hibernate's internal code (how batching is implemented in hibernate) or do you speak about
user-code using hibernate?

My understanding is that if you're using batch inserts, internally there's nothing that will guarantee your batch commits based on a time interval prior to hitting your batch size?

Affirmative,
there's nothing hat will flush your batch based on a time interval prior to hitting your batch size!

But on the other hand I also cannot figure out, for what purpose hibernate should do something like that.
Usually applications do use at least READ-COMMITED isolation level,
so other transactions cannot see the modified data until your transaction is commited.
So it this sense it changes nothing if your batch is flushed earlies or later!

N.B.: when the batch size is completed, the batch gets flushed (transaction continues) not commited!