log lenght

For summary data, all data is kept forever. The interface only shows you the last 30 days.

For individual searches, The last 500-600 searches are kept. You can increase this number by adding a line like this to your functions.php file:
add_filter('search_meter_history_size', function() { return 50000; });
However, the interface shows no more than the last 500.

I will try to come up with a nice way of allowing users to view stats for longer periods.