Hello Daniel Kussin
If you want to set a date/time limit and want to expire the link after that, you need to write a custom module for that.
Steps to achieve what you want are:
1) Create setting (under System > Configuration section) in the module for expiry (number of days) or create a custom product attribute for only downloadable products (you can do it from admin panel).
2) Set a cron in your module which will run daily and check the expiration date by adding expiration days to purchase date (for each order in which a downloadable product is ordered and the download link is active yet). If the current date is bigger than or equal to the expiration date, set the link to be inactive.
I hope my answer is clear enough to understand.
Extension in Magento2
URL : https://www.purpletreesoftware.com/downloadable-products-pro.html
URL : https://drive.google.com/file/d/1W3PzZfadLgIhcQZBeBHV4KbYWRh0lW6e/view
This the M1 extension, you can get some idea from this
Url: https://github.com/otecco/magento-product-expiration
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