Interesting comments.
The weather was fine when the student departed from the home field at both of the airfields intended to be used for the trip. Both airfields were phoned and both reported normal operations, albeit the one that was subsequently fogbound reported slight haze but "we are still flying with no problems".
The route took the student along the edge of the fog layer so he was still perfectly in sight of the surface but could see that the fog was getting worse. The airfield in question had patchy fog and so quite rightly the student made a decision to divert to the next airfield (in the opposite direction to the fog!).
It seems to just be bad luck, the initial leg of the journey had the student flying northbound and on the next leg (to the first airfield) he was flying southbound and could see that the fog was getting worse.
So, no I don't think it was bad supervision or bad judgement on the part of either the instructor or the student, it was just one of those things that happens. The entire flight was carried out both safely and legally, and I may add in full sight of the ground at all times!
Back to the original question though! What is this that some people say about 24hrs??!?
Best regards
Jimmydacraw