how long does it take to replace the hard drive

Hard to say. Maybe two to three hours.

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Normally, it takes some days but less than a week. If the hard disk your computer needs is in stock, the change will be faster and you may have your Mac running the same day or next day

The two to three hours means work time. It does not mean how much time you will wait until you can pick up the computer. That time, as mende1 states, is longer and will depend upon how busy the the service people are. 🙂