Digital BW, The Print

I know there are a few meters that work in conjunction with a 
computer, so in theory you could monitor the amount of light during a 
very long interval. Check the Jensen tools catalog, they used to 
carry things like this. Also in theory you could calibrate either a 
photovoltaic cell or a photoresistor and an A to D converter to 
report back to a computer. You could also set up some photovoltaic 
cells to run an hour meter, the lower the voltage, the slower the 
meter runs, the higher the faster (etc.). To do it correctly, you 
also need to know the spectrum of the light in addition to the 
amount, now you are talking much more money.