Ideally, you should pause the download in NZBGet (click on the big green arrow at the top left) a minute or two before the 3 hours are up. Then check in Free-Usenet a few minutes later to see if your account has expired, renew your free account, enter the new password in NZBGet’s server settings, and resume the download.

If you don’t pause the download in time, NZBGet will think there is a problem with files missing on the server, rather than realise that the server password is no longer valid. Consequently, the health of the download will slowly reduce from 100%. You can nevertheless go through the above procedure to resume the download, the only difference being that NZBGet will have to repair the missing articles at the very end by acquiring some Par2 files.

If the health ever drops below the critical value (usually 90%, assuming there are enough Par2 files to repair 10% of the download), then NZBGet will mark the download as “Failure” and move it to the “History” tab. At this point, follow these steps to get things moving again:

Renew your password in Free-Usenet for another 3 hours.
Enter the new password in NZBGet’s news server settings.
Click on “Save all changes” followed by “Reload NZBGet”.
Click on the “History” tab and find the failed download.
Click on “Failure” and select “Retry failed articles”.
You should now find that the health is back to 100%. If you can keep it that way, NZBGet
won’t need to repair any files at the end (as long as they are intact on the news server).

I have recently been in touch with the developer of NZBGet, but do not yet know whether this issue is something he can fix. (The fault may lie with Free-Usenet, depending what error message it is sending.)

Regarding your final point, the download speed for a free account is 1 Mbps ~ 0.42 GB/h (*).
That’s about 7.5 GB per day if you can renew the password every three hours while you’re awake. It’s certainly not very fast, but it is free! Much faster speeds are available at a cost, of course. For a large file such as 4K77, you’ll probably need to spend $5 or so for a fast “block” account unless you don’t mind waiting a week or two.

(*) This assumes the usual decimal convention for transfer speeds (1 Mbps = 1,000,000 bits per second) and binary convention for file size (1 GB = 1024^3 bytes).

JEDIT: I have added section 5 to my guide with the above details on how to proceed after expiry of the Free-Usenet account.