I thought it was only limited to the age of the most recent post in a pm.
If the most recent post in a pm is older then 14 days, it will be automatically removed.
From the wiki-page: http://wiki.erepubli...ex.php/Messages
Your messages to other citizens are shown as conversations. When you post a message to another citizen and he/she responds to you, it'll appear under the same message you posted. Every time you use the message form to answer in page where you read the message, the answer will be shown under same conversation. If you send a new message (from profile or new message button), new conversation will be started. Only conversations that are less than 14 days old will be shown in the inbox.
If you want to write a message to someone, you need to press new message button in his/her profile. You can use the "new message" button found from messages inbox to send messages to your friends.
- Tip: You can also create new message to your friends with SHIFT+N keyboard shortcut.
- Tip 2: You can send a mass PM to a maximum of 30 friends you have.
Please note: Citizens with Young Title are only allowed to send 1 message every 15 minutes.
As far as i can tell, there is no maximum amount of messages in a message box.