how many days minimum should I leave the old URLs active
An infinite number of days.
before I ditch them entirely
Never ditch them entirely.
Once you create a URL, it is permanent. You are supposed to keep responding to requests for that URL for the rest of time in order to avoid breaking the Web. If you want to remove the content, you still respond to the URL at the very least with a page that says the content has been removed. If you want to move that content to another URL, you provide a redirect at the original URL.
The reason for this is that the URL’s you are considering to remove are in user’s bookmarks, they are in search engine indexes, they are in links on other pages on the Web. If you don’t continue to respond to the URL’s you created, you break the Web. You penalize other users for linking to you. You penalize search engines for indexing you.
That is why you should not create URL’s casually, or move pages from URL to URL casually, or have multiple URL’s for the same content (i.e. mobile and desktop) and so on. That is why URL design is so important. That is why clean links came into being, so that you don’t have URL’s that end in “.aspx” that later change to end in “.php” because you changed server platforms. You want to just have a URL like “/about” that can remain unchanged in perpetuity because once you create a URL you cannot uncreate it.