The one and only time I donated, I could feel the effects for about a week with respect to exercise (it was winter and I was running rather than biking). My heart rate rose more quickly at the start of my run, and my overall time for my regular run was slower. I'm afraid I wasn't really charting my performance in the context of giving blood, and can't now since I was told my blood is no longer welcome.

I was a ski patroller for several years at an extremely large resort, and had been inoculated against Hepatitis, among other things. My blood shows that I am immune to Hep, and am not a carrier, but I have been exposed at some point (badly injured patient with Hep, apparently), hence they will not accept my blood.

don't go out drinking the night of, you'll be a lightweight, but I usually rest from the workouts a day when I donate (about as often as they'll let me)