If you are taking a New York Airport Service bus back to LaGuardia to pick up the rental, tell them you want to get off at the Delta Shuttle (Marine Air Terminal). More importantly than the fact that you can walk to Hertz from there, it will be the first stop the bus makes, so you can get off that much quicker.

For a 5:30PM flight on Monday, you should be very safe catching a bus around 3:30PM, with a large margin for error. While traffic is always bad, it is lighter in the summer.