How long did your resident cat hiss at the new one before it stopped meaning anything?

I brought home a nine month old about three weeks ago and my resident cat is seven and has never lived with another cat. I've done the separate room, swapped bedding both ways, fed them on either side of the door, and they'll both eat right up against it now. I know hissing isn't the same as fighting and I'm not expecting them to be curled up together by next month. I can't tell what to make of the gate sessions though. She hisses once when she sees him, holds it for about a minute, then walks off and ignores him for the rest of the time, and I don't know whether the walking off is progress or her writing off the room.

How long did yours keep it up?