Volume Control setting corresponds to how many db down (-db) from 0db at full volume?

WAY too many variables, from system to system and from recording to recording.

You could get a watt meter.  These things sell for about $45 and are used by RC airplane fanciers and others that need to make measurements at modest power levels without affecting performance while taking the measurement.  A friend and I used his old fashion needle and dial watt meter to see how much power his speakers were drawing.  He rarely used more than .1 watts, but, then again his speakers are about 105 db/w efficient.

Thanks guys and I recognize there are many dependent variables. And thanks larryi, for I can put a steady 440 hz test signal from the Cardas record through the system at the db level equivalent to what I typically listen to music at as measured on my trusty Radioshack sound pressure meter. It wouldn't surprise me that when I'm listening though my 93db Brines Acoustics FT-1600 MkII's to music volume about 90+ db, that the draw is only a watt or 2 from my 25 watt triode monoblocks.  

No never good luck.

I’ve wanted to be able to estimate how many watts my amps are driving as it corresponds to my preamp’s volume control setting. More specifically with a typical volume control having a rotational range from say 7:00 o’clock around to 5:00 o’clock, (my Croft preamp actually goes from Noon to 9:00), roughly where would it be -3db or -6db, etc. In one system with my 50 watt push-pull pentode amp, I rarely go past 10:30 o’clock. With my 25 watt triodes in my other system I rarely go past 3:30 o’clock on the Croft. Interestingly that’s roughly the same degrees of rotation!? Can I estimate approximately how many watts I’m operating at in each system simply based upon the volume control setting?
elunkenheimer

I'm sure there is a clever way to do this estimation but it's not coming to me right now.
i'm thinking out loud here & if i end up writing gibberish please excuse me....
most volume controls are 40-45dB in attenuation unless you paid a premium for your unit or the volume control in which case you would get something like 60dB of attenuation. I also believe that you would have something like 24 steps in the volume pot going from full attenuation of -44dB --> no attenuation of 0dB.
so, now you can calculate how much you've attenuated the preamp signal going to the power amp.
even if you know this, you still don't know what amplitude signal is being fed into the power amp input. You could measure this at the inputs of your power amp (probably need to pop the top lid & attach a voltmeter (which will give you the average voltage). 
Even if you know this you don't know the transfer function of the power amp i.e. what input amplitude corresponds to what output power. It's not linear; it's most certainly logarithmic. 
One of my amps has a VU meter & when the volume knob is set to -48dB, I see that i'm using 0.1W (of 120W/ch) on average just eyeballing it.