Washington (CNN) California's high-stakes primary takes place Tuesday, but it could be a much longer wait compared to other states before the winners are known.

Democrats and Republicans say they are bracing to wait days or even weeks before knowing the winners of some of the most closely contested contests, particularly House primaries.

The first votes tallied will be vote-by-mail ballots counted in the 11 p.m. Eastern time hour. Election day results will begin flowing into the California secretary of state's office in the hours that follow.

However, there will be many -- perhaps millions -- more votes to count as they arrive in the mail at county elections offices on Wednesday, Thursday and Friday.

To be counted, vote-by-mail ballots only have to be postmarked by Tuesday. They can arrive as much as three days later.

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