Interesting question. I too have reached that point where I didn't seem to get any higher as I kept smoking. My guess is that I probably was getting higher, but being that stoned couldn't really perceive the changes anymore.

This makes sense if you think about it. Especially when you consider the fact that as you continue to smoke, the percentage of increased thc in your body gets smaller and smaller with each hit. If you've only had one hit and take another you've doubled the amount of THC ( increased it by 100%). If you've taken 10 hits though and take another it's only an increase of 10%. The more hits the smaller the increase and by that time your so baked it doesn't matter anymore.