Most spinning classes are for an hour although there is one at my gym for 1 1/2 hours. Intensity level in those sessions is as you want to make it but most "Experienced" spinners make it quite high. I know for me at the end of the 1 hour I am just about on the limit and is a lot higher than I would be on a 1 hour bike ride. To do longer than 1 hour at that level would be like running a Marathon at 5,000 metre pace.

But like all forms of exercise-you have to build up to those higher levels. Normally takes me about 6 sessions before I can push as high as I should and when I started in January I was pushing at 75 to 80% of my max level and I have now worked up to 80 to 85% for most of the session with the occasional rest down to 70% and a few occasions up to 95%.

I would not like to spin for longer than 1 hour. Although you can get out of the saddle for a butt rest and you can stretch the back or use the legs in different ways- It is not like riding a normal bike. You don't get the body rests that you can take on a normal bike and it is no way as comfortable.