Lots of valid comments here, but your body creates it over time. Until you're 100%, you are more susceptible to infection, crave an iron-rich diet, and become tired more quickly. After a while you just realize you're back to normal and forgot you ever felt weird.
For me it took 14 days before I had full energy, and 30 days before I quit getting tired easily.
It regenerates to previous levels but the time depends on the amount you donate. For instance a small donation from the pelvis regenerates in few weeks.
OP here, marrow continuously grows back I believe. Marrow is basically the formation of your blood cells and those are some of the fastest dividing in the body. Up to five percent of the marrow should be unformed, new cells called blast cells.
Is it something that is limited in quantity and once it is gone it has left your body completely? (it won't regenerate to previous levels)
Perhaps any experts can shed light here and help break any misconceptions about this.