About 9 miles...if by car around half an hour including traffic if you take I-278 W.
Assuming that you mean LaGuardia Airport in Queens, New York and not LaGuardia, Spain, it takes one hour and nine minutes to drive to Newark, New Jersey.
The Hudson River - this answer is wrong. The Hudson River is on the West side of Manhattan. The East River is on the East side of Manhattan. Laguardia airport is actually on Long Island. The East River still flows past the north side of LI where LGA is located.
It is about 4.5 miles (7.24 kilometers). Brooklyn and Queens both occupy the western end of Long Island, and Queens is on top of Brooklyn. LaGuardia Airport is at the northern edge of Queens. The northern edge of Brooklyn is roughly 4.5 miles/7.24 kilometers south of LaGuardia.
It doesn't matter. All three of the major airports in the New York City area -- LaGuardia, JFK and Newark -- are roughly equidistant from Manhattan. LaGuardia is the closest by maybe five miles.
This is a bit of a tricky question. JFK is much further south than LaGuardia, making it much closer to downtown Manhattan, north/south-wise. But JFK is also much further east than LaGuardia, making LaGuardia technically closer as the crow flies (meaning, if you were to draw an imaginary, direct line between the two). Basically, if you were to draw a straight line west (northwest, actually) from LaGuardia to Manhattan, the line would hit upper Manhattan (at around 116th Street or so). If you were to draw a straight line (north)west from JFK to Manhattan, the line would hit lower (downtown) Manhattan, a little bit south of 14th Street. But since JFK is much further east, the drive in miles from LaGuardia to downtown Manhattan is still a bit shorter than the drive in miles from JFK to downtown Manhattan.
LaGuardia Airport is located in northern Queens and south of the Bronx. It is a short bus/subway trip to most of the city. Beware that there are more flight delays started at LaGuardia than anywhere else in the country.
The airport code for Long Island MacArthur Airport is ISP.
Probably because the mob boss of the Transit Union and the mob boss of the Taxi union worked out a little "wink wink" deal so that there would be no trains from LaGuardia to Manhattan. It's the only reasonable explanation. As a result, everybody flying in and out of LaGuardia gets the "privilege" of paying $40 for a cab ride and sit in long traffic jams.
5 Hours
There are four airports on Long Island. LaGuardia, JFK, Long Island Republic Airport and MacArthur Airport. LaGuardia and JFK are in Queens. Queens is one of the five boroughs of New York City, but, it is on Long Island (as is Brooklyn, which is directly below Queens). MacArthur Airport is in Islip, and Long Island Republic Airport is in East Farmingdale.
3 hours 22 mins
Even in good traffic conditions this can take 30 minutes. At peak times this can take well over an hour.