The Master tech I worked with told me that the MT fluid really only needed to be changed every 15k. I trust his judgment on most thing related to basic service.Should be immediate. You are removing the fluid which has lost its lubricating properties, and putting in new fluid.
In a manual transmission, they say you should replace your tranny fluid every other oil change, though I think that is quite overkill. A better time period would be every 4 changes, or 12k miles.