On short call, reasonably available is generally considered to be at one of the airports within 2 to 3 hours. Unforeseen circumstances (weather, bridge closings, etc) will reasonably cause that time to be greater. My understanding is that you're expected to be able to be reasonably available to one of the airports. It might take more time to get out to EWR in another words or vice versa.

The trip kit stays with me as a reserve pilot. Also don't forget the Marine Air Terminal at LGA is a little more of a complication/hassle when trying to commute into the main LGA term with a trip out of the MAT.