Seven days, no argument.
Ask from your account: open the order and use Request a refund. Or write to support@pasttofutureplugins.com and say which order.
Inside seven days of purchase the money goes back to the card or account you paid with, automatically. Your bank decides how long it takes to appear, usually a few working days.
When a refund goes through, the licence stops working within a day and the download link closes. Anything you made with the plugin is yours and is unaffected.
Outside seven days, ask anyway. A person reads it.
If a plugin does not work on your system, tell us before asking for a refund. Most of it is a five minute fix and you keep the plugin.
Buying a digital download means agreeing that the automatic right of withdrawal ends once the download begins. The seven days above are offered regardless of that.