You can increase the expiry time by increasing the value in the “” field of the settings menu but you can’t have download links that doesn’t expire.
Whats the point of having an encrypted link that doesn’t expire? it’s same as a normal link so why not just use the direct link?
If you want to give free downloads to your users using this system so the users don’t know where the download is coming from then you can use one of the two options
1) Use a squeeze page type form (http://www.tipsandtricks-hq.com/ecommerce/how-to-make-a-squeeze-page-using-the-wordpress-estore-plugin-126)
2) Use the following shortcode that will put a “Download” button for a particular product which the visitors can use to download the item.
1 is the product id of the product.