I suppose you can mess with the date system a bit. In levelup.php this code controls if a user is within the 24 hour mark or not:
PHP Code:
if($checkdate == $todaydate){
//We've gone this far, this means that we found a match because a date was returned
//Cannot vote because we already voted today...
$article_title = "You already voted for me today!";
$article_date = date('Y-m-d');
$article_content = "You already voted for this adoptable today.
Please come back in 24 hours to level up this adoptable again.";
}
This code appears twice in the file. This is what you would edit to change the time restriction. It probably won't be easy though.