Number Of Hours Of Battery Life Running Screen Saver?

Your screen doesn't need any saving, so leaving the screen saver running serves no purpose and uses power for no reason. It's smarter just to put the machine to sleep whenever you aren't using it.

"the hassle of waking it from sleep when using the MBP in clamshell mode"

What hassle? Just press any key on your external keyboard to wake the machine.

You can't run in clamshell mode on battery power anyway, so your AC adapter is always connected when you're doing so. Your battery shouldn't drain below 95% as long as that's true and the machine is idle. Are you saying that with the AC adapter connected and nothing running but the screensaver, your battery drained most of the way in a matter of hours? If so, are you quite sure no applications were running behind the screensaver?

If you're trying to run in clamshell mode without AC power, that's why you're having trouble. Clamshell mode requires AC power.

I've never seen that behavior, and my MBP is in clamshell mode nearly all the time. I don't know what to say about that except that it isn't working right at all. It should be going to sleep again as soon as you close it after waking it up, and if you're using it properly in clamshell mode, you should always be able to wake it and get a display on the external monitor simply by touching a key on your external keyboard. It should never be necessary in clamshell mode to open the MBP display and use its built-in keyboard to wake it.

When the machine isn't in clamshell mode and is running on battery power, the default behavior for waking from sleep is that the machine wakes automatically when its closed display is opened. If you've changed that in the Terminal so your machine doesn't wake on opening, as I have, then it should wake on any keypress. If your machine does either of these things when running on battery power, there's no reason to invoke the screensaver. If it does neither, something is wrong with it that needs to be fixed.

When the screensaver is running, the machine is not idle. The screensaver itself is using the hard drive, the CPU and the GPU, and whatever applications you have left open are continuing to run behind the screensaver, using whatever hardware resources they require. All these things require power to run, and the battery will be drawn down while they're running. Piling the screensaver on top of whatever else is running just speeds up the drain, and running only the screensaver (after quitting all other apps) will always draw more power than putting the machine to sleep.

I take my machine out of clamshell mode and run it on battery power for an hour or two once or twice a week. For the first 2.5 years, it was used much more on battery power than in recent months (I got an iPad three months ago and use that for most of my portable computing now). My battery is 3.2 years old, has about 200 cycles on it, and has about 85% of its original capacity remaining, so it’s in fairly good shape for its age. I expect to have to replace it sometime this year, because batteries of its vintage generally last 2-4 years or 300-500 cycles, whichever comes first. Mine has probably deteriorated more from old age than from cycling, and that will continue to be true.

Your battery is rated to retain up to 80% of its original capacity after 1000 cycles, and no one has had one long enough yet to have any real idea of its likely lifespan in years. Unless you cycle it daily, which is very heavy usage, I suspect yours too is much more likely to die of old age than excessive cycling.