According to https://en.wikipedia.org/wiki/Type_II_supernova, the actual explosion is a few seconds long. 99% of it escapes as a neutrino burst. However about 1% of the emitted neutrinos get absorbed by the start, and that causes the visible explosion.

According to http://astronomy.swin.edu.au/cosmos/T/Type+II+Supernova+Ligh... it takes a few hours for the expanding star to cool down and expand enough to hit maximum visible light. And then the decay thereafter is very slow.