Iowa hosts the first Democratic caucus, but the first deadline isn't in the Hawkeye State -- it's in Georgia. Candidates have to submit a letter of their candidacy to the state parties by October 29, though the state's executive committee can choose to include candidates after the deadline on a case-by-case basis.
The Georgia primary is Super Tuesday, March 1, which is critical if this becomes a delegate fight.
Then, several other deadlines are looming in November: For example, Alabama is November 6 at 5 p.m. Candidates must submit 500 signatures from across all Congressional districts.
Arkansas is November 9, followed shortly after by Texas, Michigan and New Hampshire. South Carolina isn't until December.
He could, of course, wait and allow these deadlines to go by. But that would be a critical mistake if this becomes a delegate fight -- like the 2008 Democratic primary did.
Qualifying for the ballot, however, is not always simple. Most states have requirements to get signatures from all congressional districts. Some states are easy -- just send a check -- others are more complicated. (In 2007-2008, Hillary Clinton's campaign almost missed Virginia's deadline because they didn't have enough signatures.)
If Biden is serious about running, he needs to express his interest before the end of next week.
What if he decides to not decide now and jump in later? It would be a risk, but it's possible: Iowa caucuses have no deadline. He would miss other delegates, but could still be a candidate.