Thermodynamics:

The relationship between heat, work, temperature, and energy. 

This is especially relevant when it comes to water cooled spindles. 

A body will always want to maintain an equilibrium with its environment. An example getting in a pool or hot tub the cold or heat will transfer to you and you'll feel hot or cold/cool because of it. 

Why is this relevant?

There is no published normal operating range for the spindles that we sell, not from us & not from the manufacturer because a lot of considerations have to be taken into account before saying "this is normal". However, the author, being in Arizona, has taken note of certain things and is able to run their spindle in an insulated garage (and before that an uninsulated shed) in the winter longer than in the summer without adding external factors (ice bottles) for cooling the coolant. The temperature of the coolant should be cold enough it is cooling, but not so cold that condensation is forming either on the hoses, connectors or the motor, nor the bucket. Humidity also will affect this, but being in a dry climate that is less of a concern for the author, but it might be something you need to consider.

The second law of thermodynamics is the most relevant in that heat will always transfer to cold and because of that the heat generating components will cool. If your coolant isn't cooling, it isn't working, and if it isnt working for long enough the chances of something breaking only increase with time. 

The warmer the coolant, the more ineffective it is at cooling because the hot operating temperature isn't being effectively transferred to the coolant efficiently.

Also, the warmer it is, the faster the temperature will climb, the higher it gets the more external factors it will take to get it under control, or the longer it will take cool naturally. 

To illustrate this, throw an ice cube in a cup of water, and watch it melt. It will take a few minutes. Now take that same amount of boiling water and throw the same ice cube, it will melt almost instantly, or at least a lot faster. 

In summary, we can't tell you what temperature to keep your coolant at, but we can tell you need to keep it cool enough that it cools without condensation forming in the motor, tubes or connectors and not warm enough that it doesn't cool at all. If you touch the body of the motor and its hot, your cooling is inefficient.