What do you gain by limiting the download?
Force people to repurchase your product if for some reason they lost what they downloaded or if you release an update.
That is seen as bad practice that even goes against Itch's guidelines.
Personally, I would never buy anything from someone who makes that kind of limitation and surely many people are going to give you a bad review for the same thing.
PS: By the way, as "redonihunter" says, what option are you referring to exactly? I don't remember that itch does have any option to limit the download of a game you purchased.
I think maybe you are referring to the download key & URL, if so, it will be the expiration of the key, in other words the time the person has to redeem the key or URL for the game.
Once you have redeemed, you can download the product unlimitedly.