How long do bans last for

They can be permanent depending on what you did. I can’t remember if support will know you might have to contact the terms of service team to find out. 

But how can you get unbanned

If you have been wrongly banned you can appeal it to the terms of service team. If you did whatever they said then the ban will probably stand. 

redbus2011 

As RadioShaqs says, the severity of bans varies, up to a permanent ban.

We cannot assist with bans directly here on EA Forums.
 
If you feel you have been incorrectly banned, you can dispute any action against your EA account by following the steps in this article: 
 
https://help.ea.com/en/help/account/information-about-banned-or-suspended-accounts/