The wake sinks slowly and is less destructive the longer the time passes...
Two aircraft 1000ft apart but with apparently no other forms of separation will not cause the bottom aircraft to encounter the wake from the above. More likely to be effected by sinking wake when 2-3 minutes behind and 1000ft below. It is rare but not unheard of.
But As far as I know there are no wake turbulence separation where a vertical standard exists unless one or both aircraft are super-sonic.