10 years
If the facility doesn't have dorms, i.e. where a single traveler can book a bed in a room with at least one, but usually more than two other single beds (bunk-beds & or standing beds), then according to the widely-held definition (including, but not limited to wikipedia) it's NOT a hostel. It's as simple as that.
Of course the necessity for a hostel to provide dorms arguably includes the factors that other contributors to this thread have already mentioned, such as:
lower cost for a bed in a dorm than a private room; social aspects of meeting others in the same dorm-room, and the possibility to make new friends, travel companions or exchange experiences
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