It’s something allong the lines of:
0-2 weeks after donation – your body excretes as many red blood cells from the bone marrow as it can to replenish the ones it just lost, after 2 weeks you should feel normal unless you’re being accurate and using a power meter in which case it’ll still be a bit low.
2-4 weeks – the marrow is still puping out new cells, but the rate slows down as the nutrients needed run out (iron and proteins specificly) and the difference between the ammount you now have and the ammount needed is reducing so there’s less stimulous to release them. Aerobic capacity should be back to pre-donation levels after 4 weeks.
4-6 weeks – the marrow is replenishing it’s stocks of nutrients and part formed red blood cells. By 6 weeks you’re back to normal.
The 12 week gap between donations is just to be sure, in some countries IIRC it’s as little as 8 weeks but they have a higher proportion of people turned away as they’ve not recovered from the last one. Guess that’s a balanceing act between getting as much blood as possible and not wasting peoples time so they don’t come back.