The Laws of Prayer (91)

1. The person responding Amen should not wait, but should respond Amen immediately after the one making the blessing has finished. (Shulchan Aruch 124)

2. One should not respond a “short” Amen, but instead it should be slightly longer. It should take the length of time it takes a person to say “Keil Melech Ne’eman, but one should not lengthen it for too long, as the enunciation of the word cannot be heard clearly when people lengthen it. (ibid)