i think one every three days at max (in terms of days), and tell the person who is next that their photo of choice is going to be next during the current critiquing.

Also, i dont think it should necessarily be a new upload.  Say for instance someone put up a shot and only got one comment saying "not too sure about this one, it seems a bit off somehow", giving them the chance to post up here asking for a more in-depth critique would be a good thing.
With that in mind, maybe the person could just give the link by pm to the mods and they put it up in the forum thread when it is their turn, that way it makes it easier for people to not miss their place in the queue.

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography