The reason is that in several places in the script the login cookie is set to:
setcookie('ow_login', $loginCookie->getCookie(), (time() + 86400 * 7), '/', null, null, true);
which is 7 days. and the last value there 'true" is for http only access, and may not be supported by all browsers, it is a security attribute and i would not recommend changing it.
I dont know about php... But just suggestion for next oxwall update maybe can still login after close browser... Like facebook, twitter, or other script i still login after i close browser... In my browser not 7 days but 5 minutes i close browser and back again to this forum or oxwall script i must login again.. Thanks...