I'm not sure it is as simple as that. I had a flight on hold recently and called before it expired to change to a different routing (rather than doing it on-line because I knew there was only one seat at the right fare basis on the new flight and I didn't want to risk losing it having canceled the other one). Without prompting, the agent put the new flight on hold for 72+ hours (end of third full day), but warned that the fare would only be valid until the end of the following day. Therefore, they can't be directly tied together.