No official dates have been released and anybody who has had conversations about
prospective dates would be covered by NDA and wouldn't be allowed to talk about
them. Having said that, it depends on how long your Software Assurance term is.
Given Microsoft's track record I would assume that the next version  of MOSS will
be available sometime next year. So within 12 to 18 months.

Historically, Microsoft attempts to ship
each new SharePoint/Office version  within 3 years of the previous to
make sure it falls within a 3 year SA agreement. The only public
information out there on availability is a comment from Steve Ballmer a
couple months ago that it would NOT ship in 2009, which seems fair
considering it's not even in Beta yet.

blogs.msdn.com/.../microsoft-sharepoint-14-is-n\
ow-microsoft-sharepoint-2010.aspx