You have directed your question to be answered by Miami professionals although you mentioned New York City in your post. Here in Florida, when a renter leases a property they typically sign a landlord / tenant agreement which covers "reasonable prior notice." Truly, there is no statute that defines what "reasonable notice" is, that is solely left between the 2 parties to discuss and come to an agreement should notice be required. Reasonable notice should be documented and signed by all parties. In the case where there is an emergency or hazard issue, the landlord can enter without notice for the benefit and safety of the tenant and others surrounding the unit. It's best that your son get the landlord to agree to a time frame or a time period where he can enter the apartment, or if the apartment is for sale your son should specify what times the property can be shown to other agents / prospective buyers.