FPJ
Doing runtime by hand sucks. I used to do it by videoing a lux meter and taking sample readings at various time intervals during playback. This gets old in a big hurry!
Automating runtime is fairly easy if you can come up with something to log voltage over time.
The sensor setup is something Terry Oregon came up with in his huge post about testing photo diodes for looking at light waveforms.
I then used his design to start my own measurements.
This works well as long as you are careful not to let the light overpower the photo-diode and clip the signal.
The OSRAM diode is very linear as far as I can tell. This same diode is used in many of the inexpensive Lux meters on the market.
I use PICO log software that comes with my PICO oscilloscope. But anything that can sample a voltage over time will do the trick.
A computers sound-card will work - just be careful not to exceed the input voltage max or it will fry your card (ask me how I know).
Some use a logging multi-meter (kind of expensive) or some sort of software/hardware to do A to D conversions and create a time-voltage log file.
I then use my Lumen Tube to get reference Lumen levels to correct the voltages measured to the actual Lumen output.
I make the plots using Excel or Libra Office Calc.
All the Best,
Jeff