The ISPs router is falling apart again, some days are better than others. We're deciding between two ISPs at the moment and will most likely begin the paperwork to move.
Since this will be a pretty large change, and one which will be physically demanding. (2 hour drive to move all the servers!) I can't even guess what the downtime will be. The sites are very complex in how they're setup and spread across multiple physical servers, so it's not as simple as cloning the site and doing a quick cutover to the new isp with a new box.
We'll work on a strategy and see if we can somehow do this in a painless manner, otherwise expect some downtime. (With the DNS change I would imagine up to 24 hours could be the down time!!! I will again, try to make this a much smaller window, perhaps with the use of another domain for the cutover timeframe so that only a few hours will be lost. (For the physical pickup and setup!))