I go to spin classes 5-6 times a week. I go very hard. I got a HR monitor at the beginning and didn't like being so aware of my HR. I know by now how hard I am working. Comes with experience. Same thing for the road bike. I find if I follow the instructor (assuming they are good) and keep up with them then I am usually in the desired zones.

Great workout in a short period of time. I am in better condition when I road ride, but spinning gets me through the winter. I spin in the summer too if the weather is crappy for a few days in a row.

Last summer I didn't go much, we had good weather, or the rainy days corresponded with an off day that I wanted.