How long does it take for all your blood to replace itself?

I just gave blood on friday and your body replaces the pint of blood that goes missing in 24 hours. srry cant help you with dead blood.

120 days. An RBC  has the longest life span of 120 days. Other components of blood renew earlier.

If you are only talking about red blood cells, then most of them live for about 120 days. In theory, on Day 121 your blood would all be new blood, but it is possible that you will have a few stragglers that could out live the average to about 140 days

This is a quote from Wikipedia, so no guarantees on the reliability, but this is what happens to the old ones:

The aging erythrocyte undergoes changes in its plasma membrane, making it susceptible to recognition by phagocytes and subsequent phagocytosis in the spleen, liver and bone marrow. Much of the important breakdown products are recirculated in the body. The heme constituent of hemoglobin are broken down into Fe3+ and biliverdin. The biliverdin is reduced to bilirubin, which is released into the plasma and recirculated to the liver bound to albumin. The iron is released into the plasma to be recirculated by a carrier protein called transferrin. Almost all erythrocytes are removed in this manner from the circulation before they are old enough to hemolyze. Hemolyzed hemoglobin is bound to a protein in plasma called haptoglobin which is not excreted by the kidney.

As was said before....Red blood cells live about 120 days.  Some more...some less, dependant on the individual.  You are always making red blood cells, so the answer is never.  You always have older blood than other blood cells, so you never have all new blood cells.  

As far as the dead blood goes...your spleen filters out dead cells.  That is it's job.  It seperates in into waste and recyclable products.  It breaks down the hemaglobin and removes through urine and stool.  It reuses the iron particles.  In a nut shell....this is the process.