After you update to Montery how long does it take for all your TV show 1000 episodes and over 900 movies to download to your external harddrive

Depends on the type of external drive, the connection speed and where your downloading them from.

Guessing that the files are HD not UHD the total file size would be about 2TB: any which are UHD would be 4 times the size.

Assuming that a) your internet connection is 100Mb/s, b) that the files are downloaded one after another continuously, c) that you are able to download continuously at the full 100Mb /s (highly unlikely) d) that your computer doesn't lock up half way through (highly likely) and e) that I haven't misplaced any decimal points, the download time would be about 45 hours - in practice more like three days. Double that for 30Mb/s, halve it for 200Mb/s, and remember that few servers feed at anything like the speed of a fast internet connection. Downloading to an external disk as opposed to an internal one probably wouldn't make much difference as the transfer rate should be far faster than any internet connection. Should.

If you want to do your own calculations remember that 100Mb/s is 12.5 megabytes per second.

Calculating the length of a piece of string would be easier.

This is like asking how long it will take to walk from Detroit to Orlando. No way to know for certain until you do it once. Too many variables and unknowns behind your question. That is macOS, not iOS.