While amount of brightness is a subjective thing, there are objective guidelines that are helpful in many scenarios. Either
- make the average brightness of the whole image = middle grey. This is what certain auto exposure meters in your camera will do. The plus side is that it typically avoids dark images. The downside can be highlight clipping. To get the best of both worlds, expose to avoid highlight clipping, then boost exposure in raw editor. I don’t know about raw therapee but in darktable all scene referred modules are unbounded, meaning you can boost values past clipping point in one module and bring them back in range with another. The data is not lost. Exposure + filmic is the basic starting point for this.
- same as 1, but instead of exposing so the average of the scene equals middle grey, expose so only your focal point equals middle grey.
In darktable, exposure module will automatically do this if you use the auto button. Just draw the rectangle around the part of the image you want to equal middle grey.
Why middle grey? Because our eyes are capable of seeing up to ~24 stops of light, but only ~14 at a time. We adjust up or down pending how bright the scene is. When we adjust, we tend to make the thing we are looking at fall in the middle (unless it is too dark - night, or too bright - the sun). So we can use exposure module in editing the same way. Alternate to exposure is curves and tone equalizer, which are more selective about which parts get brightened.