It’s funny how the taxman wants to get paid. Despite the fact that you ‘declare’ your primary residence when you file taxes, sometimes they check.
So the safest thing is to think, where do I spend most of my time? If you spend more than 180 nights in the city, you should probably declare it as your primary residence, even though you’ll pay slightly higher taxes.
It is most important to be scrupulous with this when you are dealing with two different states, because New York State does not take kindly to having its residency requirements being abused for tax purposes.
And in answer to the “how do they know?” question, if an auditor really wants to trip you up, he or she will examine your financial records to find out where you really spend your time. It’s one thing for you to vote and get your mail in the country, but what’s the story told by your ATM withdrawals and credit card receipts?