So, to sum it up, @Vogel612: It took me several hours making room for and installing Visual Studio. (Though that wouldn't really count towards getting RD ready, I guess.)
Getting familiar with GitHub, forking RD, getting VS to show the local branch, building, running tests, verifying that the newly-build RD is actually the one running in my Excel - that all took about 2-2.5 hours. This is coming from someone who has never really touched VS, so a lot of this time was spent wondering where I am and what's going on. ;) After all, it should present a reasonable baseline for newbie-contributors …