How to prolong screenshot snippets time (Mac)
I had a look at the plist for Screen Capture and there doesn't seem to be a time period to extend the time the thumbnail stays on screen. But you can disable/enable the thumbnail and/or change the default location the screenshot is saved to. For me, I have all my screenshots saved to Desktop so it's easily accessible for editing and have thumbnails disabled.
To change these two settings open the app Screenshot by searching in Spotlight. You will then get a cropped window and bar with options at the bottom of the cropped window. Click on Options and you'll see &quot;Show floating thumbnail&quot; and &quot;Save to&quot; options. You can enable/disable floating thumbnail and/or save all screenshots to another file location by default.
See image below for an example:

By the way, behind the cropped area (on the left), you will see the plist file for screen capture that I was looking at. As you can see there isn't a value to change the thumbnail length.
Hope this may help somewhat for you :)