not realistic to vote or impeach Trump before 1/20.

When will this trial get started?
That's not entirely clear. Senate Majority Leader Mitch McConnell had indicated before the vote that he will not bring senators back until the last day of Trump's term -- January 19 -- at the earliest. He said in a statement late Wednesday that the trial process will begin then.

Can the trial be conducted in a day?
Almost certainly not. This will take some days or even weeks for the group of House lawmakers who will make the case against Trump and his lawyers to answer. So a trial can't practically happen until after President-elect Joe Biden is inaugurated on January 20.