----How many milliwatts (mW) does a disc laser have in each situation? (writing,reading, speeds).
This is largely media and OPU/PUH dependent. In general, the readout power for CD/DVD is <=1mw.
More specifically, the power for reading is around 0.5mw for pressed CD, around 0.7mw for CD-R and around 1.0mw for rewritable CD-RW.
For writing the power needed increases greatly. It's around 14-65mw for CD-R, and around 6-45mw for CD-RW. Specifically, 14-20mw when writing a CD-R at 8x, 25-35mw at 16x, 40-50mw at 32x, 45-55mw at 40x, 50-65mw at 52x.
You can see that the faster the write speed, the more power is required for the OPU to successfully record to the blank disc. And the lower the disc reflectivity, the more power is needed to read the data written on the disc.
So burning at faster speed generally is not a good idea for generating high quality result and may also lower the life span of the OPU.
----How can it be measured?
Most OPUs come with a test point. You can measure the current with a multimeter.
Some drive manufacturers have their own command/instruction embedded in the firmware which enables the measurement of laser current/power directly online.
Recently I've analyzed the firmware for BenQ's drives based on Philips chipset. There're tons of strings if you open it in a hex editor. You can see lots of strings related to measurement of various drive/disc parameters. Amount these is a "measure_laser_current" command. My guess this does exactly to measure the laser power. However, I have not firgured out what the exact CDB is.