I suspect things work a bit more linearly than you might surmise. Maybe I just read your post wrong, but let me re-word it to see if I got it right, with a few changes:
Right now, we (as a human civilization) have pumped out radio signals that currently are racing out past the 100+ light year mark. This is stuff we sent long ago (e.g. Titanic's SOS call has reached the 102-light-year-mark, other early Marconi radio broadcasts in Morse code, stuff like that.)
The initial contact is the bitch - you send something out to a planet 50 ly away, hope someone is there and is capable of listening at that moment, along the frequency band you sent, has his antenna pointed at the same vector from which your signal is originating, has sufficient technology and skill to discern it as a intelligent/sentient message created intentionally. Oh, and you'd better hope something in-between doesn't obliterate the signal on its way there, and that it was powerful enough to not be diffused too much.
Meanwhile, your alien recipient not only has to receive it, but he needs to be capable of sending something in return. If he can decode what you sent and then send a suitable reply - bonus! If he sends something with the same pattern back, okay.
Now we get to wait another 50 years before the reply gets back here, we still have to be around as a civilization (with the right equipment!) to hear it, have someone interested in listening for it (what, 100 years after his grandpappy sent the original signal?), and again, hope the alien dude didn't decide that maybe a different and random (to you) frequency band would have been better to send the reply with... and toss in the same hazards experienced when sending the original request signal.
(...and you thought postal service was slow...)