Let's look at the history: '97, then 2000, then XP (2002), then 2003 & finally 2007. Looking at 2009 for the next release wouln't be too out of the question, but it's definitely not a regular schedule.

"The site made reference to Office SharePoint 2009, which hints that the next version of Office will be named Office 2009 and released that year. Microsoft has not officially revealed a timeline for Office 14."