There is no typical timeline despite what may be said by industry professionals. Most people you ask in the industry will say it can take anywhere from 30-45 days from contract signing to closing, and they would be correct, some of the time. However, New York city can be tricky. Often times because of the city's age and variations in the building construction, you can run into C of O problems, title issues, back taxes not to mention potential tenants, which can delay the closing. Now although you cannot 100% predict the timeline for each sale, as they are all different situations, a diligent agent can put forth the work to ensure all possible mishaps are diverted.
Title-insurance can be ordered the day of contract signing, a preliminary title/ lien search can take place once the property is listed, all tenants can be made aware of the sale upon listing agreement being signed, the appraisal can be ordered at contract signing . All these will ensure a smoother closing. That being said 6 Months is a bit much and providing the abovementioned precautions are done upfront, you should be able to close in 45 days. Hope this helps