Yes, this is about right; though if the target had other people fighting it a Thief (or Assassin) could spend a round withdrawing from combat and attempting to Hide in Shadows, and if successful come in for another backstrike the following round. Thus, best case scenario would be one backstrike per two rounds."I'm not sure how this is a house rule... can you elaborate?"
As I recall, In first and second edition, a backstab (that's what it was called) only worked when you actually attacked an opponent from behind who was completely unaware of your presence. So doing it multiple times in a combat was very uncommon without magic items or very special circumstance.
Give your Thief a device of invisibility and the backstrike-attempt-per-two-rounds was almost guaranteed, unless the Thief was the foe's only opponent. Cast Improved Invisibility - the one that doesn't last long but stays in place even if you attack etc. - on your Thief and the opponent was toast.