I don't put any time limit on it. When i start a campaign, I usually start to get players in advance. I open up a discord channel for them to meet, discuss, and get basic information about the campaign and setting. From this, they usually arrive at the session 0 with a character concept in mind and it then just comes down to putting the stats to the character. Using the charactermancer in game, they can create the stats within about 10 to 15 minutes, depending upon the experience level of the player and questions which come up as they are choosing options. This actually helps because newer players don't know or understand some of the options available and other players and I are there to help explain things to them. We can then also RP some of how they come together as a party, which I think helps with immersion and team building. I have yet to have character creation last more than 30 minutes which includes some of the campaign discussion and joking around. They can then either add their backstory as time allows between session 0 and the first actual play session. I have not used D&D Beyond and neither have any of my players, but I do know that there are players who do create their characters and then bring them into a campaign. Either way, it would be best to talk to the DM about their approach and how they want to have the characters made when signing up or discussing the campaign ahead of starting so that there are no surprises on both sides.