I maintain the build of a fairly large piece of software, consisting of roughly 350 csharp projects. Our build time for a debug built clocks in at about 17 minutes.
Which compiler are you using? As far as I know, up until VS2005 parallel build doesn't work for multiple projects. It's a known issue that MS keeps saying they'll fix eventually, but I have no idea whether or not they actually solved it in VS2008.