using the following declaration:

declare Event
@role( event )
@duration ( duration )
@timestamp( finished )
end

when writing the following rule:

rule "Match Any Two Events In Window"
when
$eventOne : Event() over window:time( 3d ) from entry-point "Event Stream"
$eventTwo : Event() over window:time( 3d ) from entry-point "Event Stream"
then
//nothing
end

and simulating a long run (longer than 3 days), old events (older than 3 days) are not evicted from memory. however, dropping the $eventTwo line entirely evicts events from memory properly