If you're the kind of obsessive fool like I am, and you've got a few days to spare (you'll need at least one to recover) then do it in one go. Just make sure you have plenty of coffee and snacks!
maybe i would think of doing the 24hrs in A-spec if there were things that get your attention up like weather conditions and perhaps most important: day turns into night wich will make for a fresh experience and then when you've driven through the darkness the sun comes up on the horizion and the sky is changing colors, suddenly 2 hours before the finish we get a small shower and you wonder if it's right to change to wets...
small details but they would add heaps to the overall experience of a 24hrs instead of the same situation and look lap after lap after lap for an entire day (+no good competition of the 5 AI opponents) wich is utterly boring, let's hope this will change for GT5...