Measuring lenght of time light levels peak using photoresistor
It is unlikely a photo resistor can be used to time an event with sub second granularity.  This is because the electrons in such a device have many conductive states which they migrate into when exposed to light and out of when left in the dark.  It takes time for the electrons to drift into and out of these conductive bands.  In some cases it may take a good portion of a second. As described in the 2nd paragraph in the Design Considerations section of the above wikipedia page:


  Photoresistors also exhibit a certain degree of latency between
  exposure to light and the subsequent decrease in resistance, usually
  around 10 milliseconds. The lag time when going from lit to dark
  environments is even greater, often as long as one second. This
  property makes them unsuitable for sensing rapidly flashing lights,
  but is sometimes used to smooth the response of audio signal
  compression.