Bann

My friend got banned for advertising,but we dont know exactly why and for how lont time,where can we check the bann duration ?

He will have to submit a ban appeal on the website.

Sometimes there is an automated ban system that bans advertisers (one of my old guildies was caught and took a week to get unbanned). It sucks, but no system is perfect and false positives happen, or your friend is lying.