It all depends on how much time you want to spend on it. I have about 2,000 hours of work in my Visual Studio IntelliSense projects resulting in exactly one mostly-complete language service. That said, it's not your "average" IntelliSense extension to Visual Studio - see the feature set for more info.
Here are some good resources to look at. I have a tendency to write with an assumption that users are already familiar with both the Visual Studio Extensibility basics and parsing with ANTLR. If you aren't, you should probably start at www.antlr.org and with my "ANTLR port" of one of the simple Visual Studio language service tutorials.
Here are some posts showing how serious I am about the subject. :D
PS: I can now build a syntax highlighter for a new language commenting/uncommenting in 1 day. In the same day I'm ofter able to get the type & member bars in as well.