In the US, how long before an election must  someone be registered in order to vote?

It varies from state to state and some states allow "register to vote and vote on the same day".  However, most states just require that the citizen register far enough in advance so they can include their name in the list of registered voters. That usually takes a week or so. Even then, in most (maybe all) states the citizen can still vote via provisional ballot if they registered but their name has not yet appeared on the list.  

This is the process for my state:

https://www.sos.la.gov/ElectionsAndVoting/Register...

I would type that into a Google search that includes what state you reside in as states frequently have different laws.

I think it varies by state. In my state, you can register online, and the deadline is October 5, 2020--about a month before the election. However, you *must* check the rules in your state. Did you know that Ivanka and Eric Trump were not allowed to vote for their father in the primary because of election rules in New York?

Depends on which state you live.  Check the website of the Secretary of State for your state to find the answer.

I'm not sure. Id just prepare right this minute.