Currently the mac build is done automatically on my mac laptop when the mac is on. It's done on Sunday, Tuesday, Thursday and Saturday night (Western europe time). My mac is sometimes off for a week or two or MuseScore build is broken for a period. I can also decide to stop the build because the code is changing a lot and I feel bug reports are not valuable enough. The code is now becoming more stable on trunk, I will try to provide more regular build.
The project don't have any continuous integration server available and I guess don't have fund for it. I'm not sure it would be valuable to the project to have an automated nightly build *every* night on both mac and windows. The number of mac specific issue is quite low.