In typical Pprune fashion we have moved somewhat away from the original question!
I made the mistake once of not answering the call of nature in the cruise. During the initial descent we were advised of CATII at destination with RVRs oscillating around minima. Needless to say I had no time after that and did the approach and landing bursting at the seems, which does not greatly improves ones performance. Can you imagine the scenario after a Missed Approach and/or diversion.
Moral of the story: with typical descents taking anywhere between 25-35 minutes, go just before TOD, whilst following all Company security SOPs indicated above.