On average - varying from person to person based on height and weight - an adult human has somewhere between 1.2 to 1.5 gallons of blood circulating in their body at any given time. And even though we say "blood" like it's one thing, blood is actually made of four things.
Plasma is the liquid that carries salts, electrolytes, and blood cells around your body. Red blood cells are the ones that carry oxygen, white blood cells fight infection, and platelets help us stop bleeding when we get a cut.
The interesting thing is that the parts that make up blood don't have the same lifespan. Plasma is replenished as we eat and drink. Red blood cells last about four months, white blood cells turn over about every day, and platelets last about ten days, and all those cells are made predominantly in the bone marrow controlled by hormones. The takeaway here is that blood is always turning over.
A pint of blood, a standard donation, is about 10 percent of all your blood! The plasma will be replenished almost immediately, and the platelets and white blood cells pretty soon after because the body makes those all the time, but it takes days for a stem cell in the bone marrow to become a functional red blood cell, so it can take a few days or even a week for your body to replenish all the components of blood taken for the donation.