It depends entirely on how you look at it.
Neutrino telescopes will notice a great increase in brightness on the scale of seconds to a minute or few.
Optical telescopes will take a day or few, if the many thousands of other SNe which have been observed are anything to go by.
Betelgeuse already shows a disk if your telescope is good enough. It will show an ever bigger disk on timescales between days and millennia. Compare SN1054, the outside of which is now big enough to have been seen by Messier.