Yeah, Cuomo could declare the day before the filing deadline and walk into the nomination. He doesn't need to announce any time soon.
It would be impossible for Cuomo to declare the day before the filing deadline in New York. By definition, were Cuomo to wait that long, he wouldn't be the parties' nominee, and would have to go out and get enough signatures to get on the ballot. That doesn't happen in a day - and someone would notice if Cuomo for Governor petitions were circulating sooner.
At the latest, Cuomo must declare before the NYS Democratic Party Convention, which likely will be held in May. In the real world, he has to announce sooner than that to start to build support at the convention - probably in the next few months.