I've found that it is productive to be deeply engaged in three to five projects at any given time, especially if they are of the kind that requires involved thinking and extended time spent to make progress - like writing a book. 

The more diverse the concurrent projects are, the better.

When we have been going at it on a topic for a while, we tend to get jaded and feel the need to take a break. We hit diminishing returns on the time we spend on that topic. We might get stuck and then fail to make progress no matter how much time we spend on it. 

This frustrates us and leads us down a downward spiral, which can be hard to come back out of.

Instead, if we notice that we are hitting diminishing returns and switch over to a different area that is quite unrelated, we find the vigour and enthusiasm again as we engage ourselves in something different. And when we come stuck on this topic, we repeat. 

After we do a full circle and come back to the original one that we were first stuck in, we find that we're able to see it in a new perspective and this usually leads to finding a way to get unstuck and continue making progress. 

If you're stuck in an unproductive loop on a topic, maybe it's time to hit pause on that for a while and go involve yourself in something else and come back to it at a later time.