It depends, of course. What sound are you going for? You can mic anywhere from soundboard level up, from 2' all the way out to where Cloneboy suggested. It would be very unusual to close mic a violin. Ribbons, SDCs, and LDCs are all used, and will yield very different sounds.
Can you tell us what type of sound you are trying to achieve, what the space is like, and what mics you have available?