The instrument will take longer to stabilise again if all strings are taken of at the same time (I used to be a full-time instrument repairer) but it won't do it any harm. I change mine one at a time and it depends on the guitar and type of strings... about every year and a half for my parlour guitar which is set up with Rohrbacher titanium and nickel strings; about every four months for my resonator guitar which has half-ground phosphor bronze and about the same for my mandolins which use the same. It largely depends on how much you play, the acidity and quantity of your hand/finger sweat and the preference you have for sound.