As Dave (wedivebc) mentioned above, as long as you are tracking your CNS clock daily, there is no problem with multiple day diving from a CNS perspective. Where multi-day diving comes into play is with whole body exposure (OTUs). That having been said, it would be extremely difficult for any recreational diver doing no-decompression diving to get into trouble with either CNS or whole body exposure problems as the no-deco limits will get you out of the water before either are a concern.
Technical divers, however, need to manage these issues. Especially with closed circuit rebreather diving or open circuit diving with multiple gas switches where you maintain a relatively high PO2 throughout the dive, the CNS clock needs to go into dive planning. For example, for a CCR diver with a PO2 setpoint of 1.3 the single dive limit is 180 minutes (not unheard of for technical CCR diving) and the 24 hour limit would be 210 minutes (very easy to exceed with a CCR recreational liveaboard trip of 4 one hour dives per day).
From an OTU standpoint, the worst case scenario of multiple day diving allows 300 OTUs per day. The same diver above diving a 1.3 setpoint could dive 202 minutes per day at 1.3 and stay under the 300 OTUs.
The takehome message is that the concepts of 24 hours limits for CNS oxygen toxicity and whole body oxygen toxicity are important for the recreational no-decompression diver to understand, in reality they really don't come into play. However, keeping your max PO2 under, say, 1.4 as is taught in recreational nitrox classes is very important to minimize the risk of seizures (and drowning) from CNS oxygen toxicity.
) However, IIRC the question as to whether it was possible was 
here on SB a while back, and some stated they had managed to come close on (I think) liveaboards, doing 5-7 dives a day. (I'm feeling too lazy to try to search for the specific thread.) So there's at least a little bit of basis for my original question, even if it is mostly for intellectual curiosity.
