How long do I have to rebuild a website before I lose page rank?

-Our company is rebuilding their website from the ground up. They're going to use a new CMS and everything. If they put up a sort of "splash page" that the site is undergoing maintenance, how long do they have to put up the new site/content before they lose their current page ranks? Does it matter what time of day this occurs? They currently have high page ranking but their site is outdated and they want a full redesign along with adding a lot of new information.