Is this too long to exercise on a Spin Bike?

Not too long. Now, not so sure about your plan and what you want to accomplish, where are you and where you wan to be. What are your goals?

I often ride 75-100 miles so I put more time on the saddle but not so many times a week. Yet most of my rides are about 2 hours at an average speed of 16-18 mph.

You should be doing warm up and cool down (15 minutes each). There is no logic to start up hard with cold muscles. That will lead to injury. It is much better to do the hard work towards the middle of the work out. Rather than just working hard once, is better to do intervals. That trains your heart to accelerate on demand and slow down for recovery. Also rather than work hard for 30 minutes and then slow down for 2 hours. Try to find what is the speed that you can sustain for all that time. For instance, if you work our 30 minutes at 20 mph and then 2 hours at 12 mph, you will not get as good a work out as if you do all that time at 16 mph.

But again you have to decide what your goals are: recovery, fat burning, cardio, endurance, stamina, strength....

You are doing this backwards. You should be STARTING on an easy setting which will allow your body time to warm up. You should NOT use a hard setting until you have worked up a good sweat. Use the hard setting about half way through the workout, do 30 minutes hard, then the rest of the workout can serve as a cool down. The way you are doing it, you are asking to be injured

you're doing this backwards. you should favor to be starting up on an elementary putting which will enable your body time to warmth up. you want to no longer use a stressful putting till you've worked up a reliable sweat. Use the stressful putting about 0.5 way contained throughout the workout consultation consultation, do 0.5-hour stressful, then some thing of the workout consultation consultation can function a quiet down. the variety you're doing it, you're asking to be injured

Maybe. If you do too much spinning at the expense of other exercise then it can affect bone density. Spinning is not load bearing whereas walking / jogging is and loadbearing excercise is good for your bones too.

Your body will let you know if its too much. I cant stand to do more then a hour on the trainer it's too boring. I watch TV and crank it up during the commercials.