Going my my experiences with SGA Santa and the Jack/Daniel fic-a-thon it seems that a month or a month and a half is a good amount of time for writing a 1000 word minimum fic. I think it should definitely have a min word count, but don't see any reason for a max count.
This would be so exciting to do!
If you would like any help with the work, I've been wanting to work/help out with a fic exchange for ages. I have more ideas if you're interested. You can email me at merrov at hotmail.co.uk
:D