iOS 8 has a feature allow you to set messages to delete automatically after a period of time in order to free up more storage space regularly on your iPhone and iPad.
1. Go to Settings > Messages.
2. Tap on Keep Messages under the Message History section.
3. Tap on either 1 year or 30 days.
4. Tap Delete in the popup menu to confirm you’d like iOS to delete any messages older than the specified time period.
iOS will now automatically delete all messages that are older than the specified time.