Minimum 1 weekend, be prepared for more. Occasionally things go smoothly and it's a 3 hour process, but that's rare. Make sure you have all the necessary tools, including a good light, any necessary eyeglasses, long nose pliers, Phillips screwdriver. Make sure you have all manuals for your software, peripherals, and new hardware, and that you've reviewed them. Most of all, if you can't afford to be without your computer for however long it takes, make a new one while keeping the old one intact.
Most professionals have two desktop computers.