Anything over 4 hours will loose efficacy, really depends on the program and goal. Some detox sets I have ran have gone up to 6 hour loops, without too much loss in efficacy for instance. The 4 hour rule is a guideline.
With that said, a 1 hour program will have more repeats so is going to have a more focused application and therefore yield better results (again provided correct frequencies).
If you were to put a 12 hour program across 8 generators with a 5 min. delay, then each frequency will repeat 8 times every 12 hours. However, keep in mind that those 8 repeats are going to be clustered together. So there will still be almost 12 hours between repeats, and thus too much time between and efficacy will be degraded.
Nothing wrong with going with gut feeling as a basis to start, but then also trust that gut feeling if it indicates that the outcome was not what was expected.
For more details, please check the link: