Suggestion:

on $*:TEXT:/(afk|b(rb|bl|ack)|g2g)/iS:#:{
  if (%away [ $+ [ $nick ] ] == $null) { 
    set %away [ $+ [ $nick ] ] $2- 
    msg $chan 2[4 Away Status 2] [4 $nick is now: Away 2] [4 Reason: %away [ $+ [ $nick ] ] 2] 
    set %away.time. $+ $nick $ctime 
  } 
  if ($regml(1) == back) && (%away [ $+ [ $nick ] ] != $null) { 
    msg $chan 2[4 Away Status 2] [4 $nick is now back! 2][4 Reason for being afk: %away [ $+ [ $nick ] ]  2]  [ 4 Time Away: $duration($calc($ctime - %away.time. [ $+ [ $nick ] ] )) 2] 
    unset %away* 
  } 
}