UK brides get their photos quicker, and they get the hi-res CD (when using Disney photographers).
For US (and probably Canadian) brides (and grooms

) I think they tell you anywhere from 4-8 weeks. They can come sooner, but I think they give themselves plenty of time, just in case.
Now, they were up on the website and available for all to see within a week or two.
For my announcements, we got a great pic with Goofy and used it on the front of our announcements and directed folks to the Disney Wedding photo website to view all the pictures.
I would suggest, if you want to use something for your announcement that you have someone else take a pic with YOUR camera and use it for your announcements. That would save time (and not violate any copyright issues).
Of course, this is if you are using Disney for your photography. If you are using an outside vendor, talk to them about options/pictures/copyright issues.