Impossible to answer from canon as far as I know. Moaning Myrtle's account certainly sounds like it is an instant effect.

However it is clearly not a spell in the way that Avada Kedavra is. (or we'd see green flashes flying out of its eyes constantly) I think it is something triggered in the victim when eye contact is made. A more subtle kind of magic perhaps. It seems like the brain decides to shut itself down at source.

It is probably trivially easy to stop being alive, we have just evolved to avoid doing so. The Basilisk's eyes could be compelling the brain to commit suicide while unlocking the, generally avoided, ability to do so. This has the bonus of being much more sinister than a generic killing spell, for me at least, as it uses the victim's own body agaisnt them.

Impossible to answer from canon as far as I know. Moaning Myrtle's account certainly sounds like it is an instant effect.

However it is clearly not a spell in the way that Avada Kedavra is. (or we'd see green flashes flying out of its eyes constantly) I think it is something triggered in the victim when eye contact is made. A more subtle kind of magic perhaps. It seems like the brain decides to shut itself down at source.

It is probably trivially easy to stop being alive, we have just evolved to avoid doing so. The Basilisk's eyes could be compelling the brain to commit suicide while unlocking the, generally avoided, ability to do so. This has the bonus of being much more sinister than a generic killing spell, for me at least, as it uses the victim's own body against them.