Watch the Call and Response Blues online guitar lesson by Jeff McErlain from Blues Guitar Survival Guide - Lead
"OK, so where do I come in with my response lick?" Start on beat 2 of the 3rd bar to get going, this is the most natural place to kick off the lick. "So where do I finish the lick?" That's a more difficult answer, I would say be out before beat 1 of measure 5. The more you do this the more the timing will come together. Also listen to tunes that do this, count where the fill comes in and end. I have provided some classic examples. For a real butt kicker, try doing this exercise with just you and a metronome and see if you can keep it together. That exercise is a classic solo blues and is a ton of fun to play. I'd suggest checking out Lightining Hopkins for great example of a solo blues with fills.