My rule of thumb is...when it feels right
I know that isn't much help! I do tend to go with black and white or sepia when there are a lot of competing colors with my focal point. I think black and white works well too when there is a good mix of contrast in your photo.
The beauty of LR is that you can just click through presets to get an idea of what would look good.
For this photo I knew I wanted a conversion because the original was boring so I just clicked from the top of the presets and the 2nd one "Antique Greyscale" did it for me with just a little adjustment of exposure and blacks
Not very helpful...but it is my rule of thumb!