I think they say 24 hours... so...maybe try to do a later-ish ride (assuming your appointment isn't much past their norm 4 or 5 ending time). I usually feel a little... off, perhaps a little weak, but by morning I feel fine. I think I did gymnastics once only hours after donating. I just had to be a little careful, and listen to my body. (don't be afraid to take breaks).
When I used to donate with all my friends in high school, it seemed that the smaller person you were, the harder time your body dealt with it, so something to keep in mind.
And yeah, just to echo what almost everyone has said so far... eat and drink like never before (have fun with that ).
Yay for saving lives. Way to go.
Anyway, Ive never done exactly what you're doing, so Im sure my advice isn't the best! But there's my two cents.