How long does it take to know who won the elections in the USA? (question from Portugal)?

In the U.S., the initial unofficial results are generally announced within hours on election night.

Each state, however, has slightly different rules for finalizing the count. In almost all states, mail-in ballots count if they are mailed before election day, but each state has slightly different rules for when such ballots must be received. Additionally, if there is a question about whether a person is properly registered to vote, they can cast a "provisional" ballot and most states give those voters a limited period of time to provide supporting documentation before making a final decision on whether the ballot is valid. Each local election authority finalizes their count after the deadlines for mail-in ballots and reviewing provisional ballots have expired.

For races above the county level (state legislature, state officials, Congress), one each local authority has finalized their count, they forward their numbers to the state election authority who finalizes the count (typically three to six weeks after the election depending on the state). It is only at that point that there is an official winner.

While the results are not final for several weeks, the reality is that we know the winner in most races on election night. Most ballots are either cast (or received in the case of mail-in ballots) on or before election day. Unless a race is really close, the remaining ballots simply are not enough to change who won.

each jurisdiction has its own procedures and some will have rechecks on the results, which could take months in some cases

Depends on how close it is. Most of the time you know that night. But really close ones can trigger hand recounts which can take weeks.

It was all announced last night. Look at CNN or any american news site. The BBC in England was broadcasting it yesterday too. For american news I always go to CNN first because they always have the latest updates.

For the most part, final results are in. There are a few races where it is too close to call. The Georgia governor's race may require a runoff, meaning the two highest vote getters have to go through another election.

Most races announce results the night of the election. This happens when one candidate is a clear winner.

For close races we sometimes wait until all the votes are counted which can take a few days.

If no candidates gets > 50% of the vote there can be a runoff which takes place about a month later.

democrats picked up 30 seats in the house and a few Governorships, republicans picked up a few seats in the senate. That's basically it.