But we kinda do know [how many encounters Larian expects us to fight before resting], since after some time our characters starts to complain that they want to rest ... observe that for a while and you can discover some pattern there.
My personal guess is that its tied to real time and not amount of encounters at all.
What I've heard is that these voice barks are/were used to express that a companion has new dialogue. I dislike that implementation, but it makes a bit of sense. Voice barks tied to real time, however, is super useless.
I would be in favor of voice barks where characters express tiredness after X encounters/events or you reach Y new locations.