In order to verify if they can give you the car you requested, they need to know how long you will need the car for.
"How long will you be needing the car?"
"How long will you be renting the car?"
"When do you need the car till?"
"I'll need the car for two weeks."
"I want to rent it for one week."
"I need it for 4 days."
"I need it for just a day."
They might ask you to fill out a form, or they might ask you many questions.
"What is your last name?"
"What is your first name?"
"Are you a United States resident?"
"What country are you from?"
"What is your address?"