In Winbox, I know that I can view the bytes-in and bytes-out for each users.
where Can I view the total bytes-in and out for all users? (Total download and upload from my ISP)
On the interface itself.
I think it is reset after reboot. I want one that doesn’t reset like the hotspot users
Print it into a file every 5 minutes via the scheduler, or use an external monitoring system. AFAIK there’s no total traffic counter built into RouterOS that persists across reboots.
In hotspot what does it mean to set cookies time to 7 days for example. will that record the user usage for 7 days and all counter will reset after that?
No, that saves a cookie to the user’s browser containing the login credentials he used to log into the Hotspot last time. When the user drops from the Hotspot but comes back, the login page will load again. If cookie logins are enabled, the user will automatically be logged in on merit of the cookie as long as uptime and transfer limits permit so, and the user doesn’t have to provide credentials. The timeout is the validity of the cookie.
All that is explained on the Wiki: http://wiki.mikrotik.com/wiki/Hotspot#ip_hotspot_profile
If the cookies expired, will the user be cut off and forced to enter the user name and password again.
Again, the cookie only contains login information and automates the login. So no, if a cookie expires during a user session the user stays logged in, but will not be logged in automatically the next time.