Let's say it takes ~12 hours to tally ballots for your country (results are available the next morning). We have ~330M people in the USA, so if it scales linear

Let's say it takes ~12 hours to tally ballots for your country (results are available the next morning). We have ~330M people in the USA, so if it scales linearly, the full count would take ~16.5 days. For what it's worth, most of the "long tail" time in counting votes in the US is due to special circumstances, like mail-in ballots which were damaged during delivery and need to be transcribed onto a new form before they can be run through the counting machines. Either way, projections are a fairly simple statistical exercise. Once enough votes from a district have been tallied such that you are confident that the statistical error wouldn't change the outcome, you can project a winner for that district. That is, if you collect a statistically significant sample such that your error is +/-X%, and a given candidate is winning by more than X%, you can project them as the winner.