Final answer:
The total time from receiving the initial message from a civilization 35 light-years away to when we could expect a reply to our response is 74 years, considering the time it takes for light to travel that distance and the decision-making periods involved.
Step-by-step explanation:
The question pertains to the relationship between distance, the speed of light, communication (message transit times), and versus the actual age of planets. In the scenario where a radio message is sent from a civilization 35 light-years away, it takes 35 years for the message to reach Earth due to the speed at which radio waves (traveling at the speed of light) propagate through space. When humans send a reply, we first have a 2-year deliberation period before the message is sent. Once transmitted, our message takes 35 years to travel to the other civilization. Then it would take another 2 years for the extraterrestrial governing bodies to formulate a response, plus an additional 35 years for their reply to reach us. Thus, the total time from receiving the first message to getting a reply to our response is 2 (our deliberation) + 35 (our message travel time) + 2 (their deliberation) + 35 (their reply travel time), equaling 74 years.
Once ongoing communication has been established, it is a further consideration whether to continue sending messages without waiting for a reply, which could potentially reduce the total communication time.