Session doesn't end when browser window is closed

7200 is two hours. For something that absolutely has to stay longer, you can set cookie data separately with a different expiration time, but you should probably be using a different storage mechanism than the session or cookie for longer-term data.

Realistically, you shouldn't even need flash data for the validation messages unless you're redirecting to display the form with the error messages. If you just load the view when the validation fails and populate the error messages, the data doesn't need to be persisted in the session, even for one request. (If you are populating the flash data and loading the view rather than redirecting, this may be the reason your validation messages are returning, depending on how you actually retrieve/display the messages.)

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide