If Biden/Harris loses, Harris retains her Senate seat.
If Biden/Harris wins, Harris likely will resign from the Senate effective noon on January 3rd, 2021, allowing her successor to enter the Senate with favorable seniority.
Tim Kaine didn’t resign, and he was sitting US Senator.
Paul Ryan didn’t resign while being a sitting Congressman
Sarah Palin didn’t resign and she was sitting Governor
John McCain didn’t resign and he was sitting US Senator
Barack Obama didn’t resign and he was sitting US Senator.
Given the import, presumably, of the position of the presidency even without a pandemic to deal with, it is reasonable to assume that it requires the full attention of the incumbent. Even more than for a sitting Senator.
She will resign whenever she is ■■■■■■■ good and ready to resign and not when some random internet poster thinks she should resign.
That being said, she will most probably submit her resignation to be effective at noon on January 3rd, 2021, so that her successor will have favorable seniority in new class of Senators.