You do not have to be separated from your husband for any specific period of time, but you need to have specific grounds for divorce. The most likely grounds in New York are cruel and inhuman treatment, adultery and abandonment. If there are sufficient grounds, a divorce action can be commenced before your husband leaves the country. It is possible to commence a divorce action after he leaves, but it would make things more difficult and more expensive. I suggest that you have a consultation with an experienced family law attorney.