Of course, my reply is ‘it depends!’ Most people can see clearly that an elite level athlete can tolerate many more sessions per unit time than a beginner or someone carrying a complication such as an injury. So there is no standard unit of time to rest between training sessions except this one:
Rest as long as it takes your body to recover from the specific stress you have placed on it.
This rule has two messages; the first is that you can only use the messages coming from your body to decide how much training it can handle. If your performance is going down from session to session, while you are training more than usual and feeling tired and sore, then maybe it is too much. If nothing is happening (no improvement but no soreness or temporary fatigue, then maybe you could experiment with more.
The other part is to keep in mind that climbing training requires variety in the venues, modes, intensities etc. of the training stimulus. Exposing yourself to this variety is not just important for training all the elements, it also allows you to spread the stress on the body across different muscle groups and energy systems and hence maximise the overall training load.
So, what does that mean in practice? If you go to the same wall and do the same problems, week in, week out, You will only manage a small proportion of your potential maximum training load without getting plateau and then injury.
But if you mix up your training in any way you can, you will be able to handle much more days on and longer sessions. Even subtle variety will help here – a different board, problems set by a different person, different hold manufacturers etc. But don’t use this to neglect the big sources of variety – routes instead of always bouldering (or vice versa), different rock types, different training venues, different training partners and many more…