Each instance of Customer model file, has methods like countUsageFromQuotaMark() / getLastQuotaMark() / createQuotaMark() that you can use, just look inside apps/common/models/Customer.php file to see them.
You can create a customer instance by running:
$customer = Customer::model()->findByPk(1);
or if you are in customer area and you wnat to get current customer instance, by doing:
$customer = Yii::app()->customer->getModel();
Then you can simply call methods on the $customer variable.