I routinely do not prescribe crutches for a specific length of time. Also, different physiotherapists will give different advice about this. My personal feeling is that you should use them for as long as you personally feel that you need them, and I am happy for you to discard your crutches whenever you feel you are safe to do so. This is normally somewhere between 2 and 4 weeks post-operatively but obviously there is variation between patients.