1. Go to /ow_plugins/billing_paypal/views/controllers/order_form.html file.
2. Find these strings: 

{if $sale->period > 90}
<input type="hidden" name="p3" value="{$monthPeriod}" />

and replace them with these ones:

{if $sale->periodUnits == 'months'}
<input type="hidden" name="p3" value="{$sale->period}" />

3. Go to /ow_plugins/billing_paypal/controllers/order.php file.
4. Remove the following string: 

$this->assign('monthPeriod', intval($sale->period / 30));

5. Go to /ow_plugins/membership/controllers/subscribe.php file.
6. Add the following string $sale->periodUnits = $plan->periodUnits; 

right after this string: 

$sale->recurring = $plan->recurring;

7. Go to /ow_system_plugins/base/bol/billing_sale.php file.
8. Find this string public $extraData; and add this block of code right after it:

/**
* @var string
*/
public $periodUnits;

9. Go to phpMyAdmin and run the following query:

ALTER TABLE `ow_base_billing_sale` ADD `periodUnits` VARCHAR(20) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL AFTER `extraData`;

Note: ow_base_billing_sale is the tabe name. If you use another prefix, you should replace the provided table name with yours.

That's it.