I'm saving a file url by using dropin api, after a while that url starts to give 401?

Which Drop-in are you using? (The Chooser or Saver?)

The "direct" links from the Chooser do expire after four hours, if that's what you're referring to.

Otherwise, please post some more details, e.g., a sample request/response.

Yes, i use Chooser. That means i have to take files to my host when selected...

The direct link type for the Chooser is good for four hours, so if you need to download the content, you'll need to do so within that window. If you need longer-lived links, you can use the preview link type. (Those don't offer direct file access by default, but can be modified as necessary.)