The Delete Old Messages feature on LG G5 automatically deletes text messages and multimedia messages when the message limit reaches. To delete text messages on LG G5 automatically, do as follows:
1. Open the Messaging app on your G5
2. Hit the 3-dot icon on the top right and go to the Settings.
3. Under the Advanced Settings, hit the More option
4. Go to the Storage settings
5.Toggle on the Delete Old Messages option and then specify the limit for text messages and multimedia messages.
6. From now on, you oldest messages will be deleted as soon as the limit is reached.