To literally threaten the galaxy, the supernova would have to expand beyond its initial system eventually. In which case it would not matter whether this is the Romulan home system or some other system: creation of a black hole would have to be done at the initiating system, and before the shockwave got too big to be swallowed by said black hole.
Whether the threat to neighboring or faraway systems from the shockwave would then be in terms of hours, decades or centuries would be immaterial: Spock would in all the cases only have "little time" to stop the disaster in its tracks.
And natural supernova wavefronts do threaten neighboring star systems to the distance of hundreds of lightyears. Plenty enough to qualify as "the galaxy", as that would encompass all the major Star Trek players in the modern "small UFP" model of Trek cartography. A sublight shockwave that took centuries to do its work would be just as deadly as a FTL one in this respect.