If your lease is up for renewal and you want to stay, sign and send it back right away. If you wait too long, your landlord can decide whether to consider the lease renewed, or sue to evict you. If your lease is deemed renewed and you want to move out shortly thereafter, you may then be liable for the full amount of the rent over its entire term, even though you've vacated.
Alternatively, if you want to stay but did not renew, the landlord can sue to evict you, and to obtain the lease's full term of increased rent that you should have paid under the renewal. With litigation you always risk eviction, attorneys fees and blacklisting.
If you're undecided as to whether you'll continue occupancy, consider having a Tenant's attorney negotiate a written agreement with your landlord that gives you an early right of termination, with a specific term of notice. For example, perhaps you could vacate on 90 days notice with no additional obligations. Otherwise, procrastination can be an expensive proposition when you're dealing with a New York City landlord.