You can reply to a conversation at any time, no matter who posted last.
To find the conversation you want to reply to, click on the envelope icon in the right hand side of the top menu, and click 'Show all...':
Find the conversation you want to reply to and click on it. Scroll to the bottom of the conversation and you will see the usual text editor:
Type your response and click 'Post reply'. Your new message will be added to the conversation immediately and the other participants will be sent an alert.
It's worth noting that you must leave at least 30 seconds between each message you send in a conversation - the system will not let you send more than one message in any 30 second window. This is an attempt to prevent conversation spam.