I thoroughly believe that 4 hours is the limit for most people (on most days) on how long they can focus deeply on a problem. That was at least my experience as a mathematics grad student. In math this is more evident because most of high level math requires this deep level of understanding.

Of course one thinks about these problems while doing other things (obsession is a common consequence of prolonged deep thinking), which is why visits to the restroom, walks outside and so on are famous to prove very productive.

Either way, math is also social (most problem solving benefits from discussion) and it is in my opinion much more productive to set some time off for talking about / working on stuff with others than grinding through longer. This is still work and incidentally also good time and resource management.