This depends primarily on the size and the speed of the drive.
A 1 TB drive running at about 5400 RPM takes between 3 and 4 hours to give you a rough idea. Other factors such as whether the drive is internal or external, uses SATA or SCSI or IDE will all affect the time it takes.
Fills the hard drive to 0's.
zero-fill utility
completely wipes a hard drive clean
completely wipes a hard drive clean
Zero-fill utility page 1031 Guide to managing and maintaining your PC 7th edition
The answer is one fellow by as much zero to fill the universe and them some.
A point
Fill the tank and set trip meter to zero. The next time you fill your tank divide the number on the trip meter by the fuel purchased = fuel consumption.
What I would do is fill the gas tank all the way up. Note the cost of the fuel and the amount of gallons it took to fill it. Clear your trip meter so that it is on zero. then run the vehicle for a day or two (depending on how far you drive each day) then refill the tank , note the mileage that you had travelled and divide the mileage that you had gone into the number of gallons it took to fill the vehicle the second time. You now have a estimate of the miles per gallon. Just keep track of the miles you have driven after that.
The simple way to clean and remove every data including OS from your hard drive is to perform the low level formating or zero fill task using windows 98 bootable disk. It is a time taking job but it is reliable and resonable.
dd if=/dev/zero of=/dev/hdxSubstitute the appropriate drive letter for x. Be very careful that you do not erase the wrong hard drive.
Fill your tank and write down the mileage or set the trip meter to zero. Drive the vehicle until it is at 1/4 on the fuel gauge and fill it up. Divide the gallons it took to fill it up into the total miles driven. You now know the mpg of your car.