Hi,
I'm not sure what "gets access to the products" means. Because, I suppose it's not allowing the customer to purchase the products for 0, without payment. If that was this, it would then generate a subscription based on the plan linked to the product purchased for 0. And then, after the 15 days, you could remove the "free products" mode, but the customer would still have his subscriptions running until they expire.
So I suppose that you men that you want the customer to get access to something that he normally get access to after purchasing a product and getting the corresponding subscription running.
But I don't know how you configured your subscription plans and what is given to the customer and how after he usually get his subscription.
For example, supposing that you're giving access to Joomla articles as subscriptions by adding the customer to a specific user group corresponding to the subscription, you could modify the way they access these articles to also include an extra "trial" user group which would be automatically set for new user accounts (using the "user group on registration" setting of the HikaShop configuration) and then configure a mass action to change the user group to the "registered" user group after 14 days. But I'm wildly guessing here. I would need more details on the situation to be able to say what would be best.