I have made curbside out of Terminal 7 in under 20 minutes, with picking up a checked bag (I was first to deplane, bag was the first one out)and GE.

However, getting into LGA during that time in a car these days is a sh*t show. Curbside to curbside, it will take at least an hour. So if every is done as fast as possible, you'll make it check in by the time your plane is taxiing.