I do something similar to what you described. I only work out with weights twice a week. I can definitely say that I haven't lost any muscle, in fact I like the way I look better this way. I think I have a nice *toned* look that isn't overly muscular.
Everyone is different, of course, but doing full body weights twice a week has been enough for me to maintain my muscle mass. I also switch strength training videos quite frequently(which I like to do) so I also feel like my muscle memory is constantly being challenged. HTH