Final answer:
The time it takes for a white dwarf supernova to decrease in luminosity by a factor of 100 from its peak is not precisely given, but it generally takes from several weeks to a few months for the brightness to fade significantly.
Step-by-step explanation:
The decline in luminosity of a white dwarf supernova from its peak brightness takes approximately several weeks to a few months. However, the timeframe for the decrease by a specific factor, such as 100, is not provided in the information. Supernova SN 1987A's light decayed between days 40 and 500, with the rate slowing due to energy from radioactive elements. Generally, after reaching their maximum brightness, supernovae become less visible over a period of a few months to years.
Type Ia supernovae, which are the type that white dwarfs can turn into upon reaching the Chandrasekhar limit, can outshine their host galaxies and remain visible over large distances, allowing for observation with powerful telescopes.