Someone dies of inactivity after 24 hours from the last GM post (correct me if I'm wrong though)
unless with valid reasons given to the GM.
If someone has not replied it's best to wait till the 24 hours are up and the GM kills them off. sometimes it wouldn't be exactly 24 hours as the daily updates are posted at the period during the day. Once they are killed (which will be announced by the daily updates and sometimes the next GM post will mention the death of that particular inactive tribute), you can continue to post again, your charrie won't be killed off if you are waiting for someone else to post but has also past the time limit c: Hope it answers your question!
happy hunger games~!