Usually around 38 weeks, unless your consultant/obs considers earlier medically preferable. At 38 weeks twins are considered 'term' and I think any later you might run the risk of going into labour spontaneously, which would necessitate an emergency (i.e. that day) c-section, which they prefer to avoid if they can.
If you're not too far along, it might be possible that your twins will turn, but given the space restrictions with two fighting for room, it's less likely.
Kx