You don't mention which calendar template you're using so this is from flat plus
if you look in the month view calendar cell.php search for this line
JevHtmlBootstrap:opover('.hasjevtip' , array("trigger"=>"hover focus", "placement"=>"top", "container"=>"#jevents_body", "delay"=> array( "show"=> 300, "hide"=> 200 )));
You can try to adjust the hide time there this is what I had done.