The Bay of Biscay geographically is missed by most cruise liners and they sail past that area in the Atlantic. The distance in which the rougher waters can be experienced is about 450 nautical miles and takes about 22 hours to traverse. What you have to remember is that the entrance to the English Channel and the west coast of Portugal can be equally as rough as the so called Bay of Biscay. Conversely we have been through there with very little sea running and this can happen at any time during the year but is most likely in summer. Hope this helps....Falan Dow.
I agree with the previous answer i would like to add that most cruise ships arrive in the area in the late evening so most of the time you will be in bed asleep and in the morning when you awake up you will be almost across the bay.
Approx ten hour.