When does being happy about the new year end?

Today I heard a man wish another man an enthusiastic "Happy New Year!" near the 16th & Mission BART stop and I had to stop and take pause.
And so i did.
I thought, "hmmm, that seems odd to hear someone still wishing people happy new year three weeks into February."

When does it become too late to wish happy new year? What's the cutoff point?
And should we all, as a society, agree on this together?
Or should it just be one of those unwritten rules.

It's all very confusing.
Damn.