A month might not be necessary, but if you can that's a good number. It takes a couple weeks at a minimum, and really depends on the ip that happens to be crawling you at the time and whether it has it cached.
I moved a site and had hits to the old IP for over a week, but less than two (big site, lots of crawls on a regular basis). A month is probably safe.
That's probably a good idea. I am moving a colocated server to in-house next week sometime to better defend it against scrapers etc. I need to have a finger on the firewall and telecom line to do what I need and I can't have that with the remote datacenter. I had planned on running duplicate servers for a time, but would not have probably though twice about killing the colocated server after a week or two.