How long in seconds(or ms) is each frame?

For most MP3s made from a CD (44khz 16-bit stereo), the length of a frame is approximately .02612/second long.

If it is VERY small in length then why not have the last frame of applicable songs just removed?

Yes, that's the very concept under which my GapKiller program operates. Only it's not enough just to trim the last frame. You have to mess with trimming different numbers of frames before and after each gap, and some trial-and-error is required. Have you tried it?