Hi!
The growth rate can vary both individually and with age; so can the age at which nails start growing out. There is no set time for that reason; you just check the nails during your weekly health monitoring weigh-in and body check and cut them when there is enough growth that you are not cutting into the quick. It is usually plus/minus a month.
Apart from the light trick and especially with adult/older piggies you can also judge from which point dark nails are dead by seeing growth markings appear whereas the live part of a nail is always smooth.
With babies, you'd best cut just the very tip (ca. 1mm) once the nails start growing out for the time.