Whoever Biden chooses to be VP will effectively be the president on Feb 1 2021 if Biden is elected. They will probably have the title of President by the end of 2024. I would reserve judgment until Biden announces his VP pick, however since effectively winning the nomination, Biden has been pulled further to the left by Bernie Bros, so who knows what kind of presidency a Biden administration would be.
Biden is undoubtedly going to pander to the leftist Bernie crowd and his Presidency won't be nearly as far left. I'm going to assume Biden is going to pick a moderate as his VP and if she begins to implement leftist policies that are wildly unpopular according to poll data, Democrats will get pummeled in the midterm elections two years later.