The limitation is the computer’s memory. 6 minutes is more than possible to produce.
Personally I won’t make a 6 minutes animation in a single file. Although you can do that, it is very difficult to synchronize all the animation with that length. Any small modification at the start of the animation will affect all the rest and maybe you need to tweak a lot the whole animation. Also, It is riskier to handle all in one file.
The normal behavior is to split the animation in scenes and each scene in shots and animate each shot in a separated file. Later using Synfig itself or a video editor, compose the final result in addition of the soundtrack.
Unfortunately Synfig doesn’t handle soundtracks yet.
Any average computer of 3 or less years old will be enough. There is not needed any kind of video card or CPU speed. Anyway, the more the better.
And better Linux or Mac than Windows.