In MyRepChat you can change how much message history you would like to view in your conversations. This is set to 90 days by default.
To change your message history time frame:
From the Dashboard go to 'Settings'-->'Preferences'-->'Message History' and from there you can adjust the time frame.
Click on the "Save" button once you have have made your changes.
Once you are back in "Messages" you may need to refresh your page to show the changes.