If using a MySQL database storing all downloads, just add a timestamp column
(SQL -> "alter table your_table add column TimeStamp timestamp")
This will allow you to store the moment in wich the download is asked.
Then, in PHP just:
$actual_time_in_seconds=time();
$number_seconds_in_seven_days=72460*60;
$last_seven_days=$actual_time_in_seconds-$number_seconds_in_seven_days;
$query="SELECT * from your_table where TimeStamp > $last_seven_days";
$results=mysql_query($query);
This will show you all the rows that have been inserted in the last seven days
