[Berkut]

I use simple rule - if I can see the grenade (when it goes off), I am dead.

As Tys mentioned though, there are a lot of guys who feel that a chair of sheet of plastic provide adequate cover, or if they don't "feel" the hit, they are not hit.

Good host will define rules for grenades at the beginning of the night, if not it is free for all, in most of cases.