I did some messing on a turbo trainer with a HRM many years ago, when I was a little better trained than I am now, although probably not much good for anything other than an evening "10" since I wasn't doing any long rides at all. My natural cadence, when reasonably fresh and on a bike with gears, is about 105 rpm. However, the cadence at which I could produce the highest aerobic power for 20 minutes was more like 80-85rpm.
You tend to find that the fast guys in TTs are pushing - not spinning - very big gears. Purists hate this, and it is less likely to work for a "50" or a 24, let alone distance riding like audax or touring, but it is the most efficient for an all-out effort.
If you ride fixed, you discover that cadence doesn't matter as much as people think it does! I've trickled to work in the snow at 40rpm and ridden with tailwinds at 120rpm, descending at 180rpm in I'm-going-to-die style. If it feels like you're pushing too hard and you have no gear to change down to, get out of the saddle to save your knees.