I agree with cheep, 15-20 is a bit too much but I would jump straight to a 6/8 map delay.
Looking at my gaming session on the 23rd of November:
ET Beach Invasion
It was actually not bad at all, there was a good variety of maps being voted on that day but you can see Special Delivery being voted 3 times there.
Between the 1st and 2nd time that Special Delivery was voted we had 12 different maps. The second interval shows only 5 different maps and 3 of them are Venice , Goldrush (these were both voted twice in the session) and Oasis.
We have a great variety of maps therefore a 6/8 map delay wouldn't be an issue and I don't think anybody would mind playing Goldrush/Venice/Oasis/Special Delivery/ Italy every 8 maps or so.
I think it's a good idea.