I don't really think you can tell without some tests in your own context, but for what it's worth here's a summary of my experiences when I recently moved from a 10K drive to SSD.
Small solutions/projects build in about the same time, but the solution that has the most files and complex build (it has a mix of web projects, class libraries, LINQ2SQL, postsharp, etc) went from about 4min 30s to about 3min 40s for a full rebuild in serial. With MSBuild running with the /m option this went down from about 4min on the 10K drive all the way down to 2min 40s.
Far more importantly for me though is that when working on that solution Resharper would chug along a bit when first opening it and also when working within some of the bigger source files (which shouldn't be there of course...). Now though I can barely feel any difference between working with Resharper enabled/disabled so although that would be hard to measure it's massively more pleasant to work with now.