I concur, i wouldn't take off any rounds, I'd just convert any "out" rounds to "down" rounds, and consider the character to be "Down" until enough rounds of stun wear off for them to be awake without the spell effect. I suspect that'd qualify as a house rule though.
Technically, if you wanted to be a real stickler for the rules, the character would awaken, but if you didn't then hit them with some stun relief, they'd pass out again at the end of the round.