A giant star nearing its death throes
has to go through several phases, prior
to exploding as a supernova.
Those phases and their increasingly shortened
time frames are well-known.

e.g.
Converting/fusing light elements like carbon and or oxygen
into heavier elements like magnesium, silicon, sulfur near the star's core.

Then fusing magnesium, silicon, sulfur into heavier elements still,
ultimately resulting in an iron core, halting further fusion and resulting
in the sppedy collapse stage leading to a supernova.

Can't recall how long the exact length of time for the last
fusion-cycle/stage leading to the formation of an iron-core, but';s it's not very long; a few centuries at most.

There are tell tale signs for each stage: temperature changes,
superflare outbursts, spectrum changes, etc.