Dear how long?
The problem is that there is no specific duration in a law to let you know when it safe to dispose of a tenant's abandoned or left behind personal property.
New York law may hold the landlord liable for the loss or destruction of the tenant's property. New York law imposes a duty on the landlord to safeguard the property.
That means, that your sister cannot just count down to a "safe" date. She must actively involve the former tenant in coming to deal with his property.
In a worst case scenario, your sister may need to place his possessions into storage (unless she has enough space to self-store.)
You did not mention what he said to your sister about getting his stuff.
She should communicate in writing. That is, send a letter to him, with a copy to herself, mailed from a post office, and sent by certified mail and by regular first class mail, with a certificate of mailing. Her letter should describe the abandoned property, the location of the property, and how long it has been since the tenant left the possessions in your sister's care.
If he wants your sister to dispose of his property, he should let her know. She can then donate at any number of local locations, instead of just throwing out.
Your sister may need to consult a local attorney as the danger of acting out of frustration may be costly to her.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.