I have a friend who gives blood every 6 months. He says it's a good way of replacing your blood and is of similar effect than women menstruation. which in his modest opinion help them live longer.
Perfectly true, that's why we all still opt for a blood-letting session while we are at the barbers (red and white pole...).
Load of old tosh - the body is perfectly good at retiring the blood cells that are defunct.
I'd say 24 hours was fine - last time I gave blood I went for a run 12 hours later and 5 miles in I felt a bit rough. I hadn't had a big supper after donating, and had missed my pre-run cup of tea.