Typical DNA replication times
A mammalian cell takes about 8 hours to replicate all of its DNA in its S phase; a yeast cell would take about 40 minutes.
Some other information that you seem to not have quite the right information about:

The DNA polymerase does not unwind/split the DNA—that is the job of DNA helicase. The DNA polymerase cannot bind to the single-stranded DNA until the helicase unwinds the double-stranded DNA (&quot;melts&quot;), or the dsDNA is artificially melted in a test tube via high temperature.

The dsDNA is not fully separated before it is copied. There are tens of thousands of sites—origins of replication—across the genome that unwind and begin replicating every time the cell replicates its genome.



&quot;...DNA replication in most eukaryotic cells occurs only during a specific part of the cell-division cycle, called the DNA synthesis phase or S phase. In a mammalian cell, the S phase typically lasts for about 8 hours; in simpler eukaryotic cells such as yeasts, the S phase can be as short as 40 minutes.&quot;
&quot;An average-size human chromosome contains a single linear DNA molecule of about 150 million nucleotide pairs. It would take (0.02 seconds/nucleotide) x (150e6 nucleotides) = 3e6 seconds (about 35 days) to replicate such a DNA molecule from end to end with a single replication fork.&quot;
&quot;...Approximately 30,000–50,000 origins of replication are used each time a human cell divides.&quot;

–Molecular Biology of the Cell, 6e, Alberts, et al.