It really depends on which terminal at LGA you are going to and time off day. They take you from the rental location via their shuttle, to a public bus stop, then an airport bus takes you to the terminal. Be prepared for a 20-30 minute commute.
Can't tell you in actual time. But it is a short ride from the car rental to the terminal using the air train. It all depends which terminal you have to go to. 5 min tops.
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