I used mine right from the start with the first, up til I got in the bath at about 4cm, then on and off as I was in and out of the water.

With the second I had it on from first contraction (but was already at 4 cm by then, possibly more) until I got in the bath, which was an hour or so later, so maybe 6cm? Then put it back on when I got out of the bath in transition, to go to the hospy. Could not have made it to the hospy without it!