Tested it on a few sites and it appears to be close to real-time. Still sucks that there isn't an easy way to do these browser-shot type of tools with sites that aren't live on a url somewhere though. Not much to be done about it I suppose.
You mean sites that have not been publicly launched yet ? As the other comment points out you can install the browsershots code - it's open source. It has a stand-alone mode which doesn't even require you to install the server. Contact me off-line if you require any help in setting up the browsershots.