I have donated blood (and more recently, I've been donating platelets, but that's completely different) very regularly for several years. For the next 2-3 days after my donation I try to stay off the bike and the weights, but I start again after that.

I find that it takes about 7-10 days after the donation before I feel like I'm riding at my normal levels again.
As Davet quoted, you actually recover your lost blood a lot quicker than the 8 weeks that you typically have to wait before you donate again. You typically recover 100% of your lost red blood cells around a month after donating.

The first 24 hours after donating are the hardest for me. If I let myself, I can take a 5 hour nap immediately after donating.