I really think it depends so much on the individual. I can go do anything right after and I'm fine (after a glass of tomato juice), others feel down all day.
I can't donate anymore*, so I've volunteered to herd the cats - er donators. I'd say only about 5% have serious faint-like symptoms. Another 10% or more are like me and unfazed.
The rest are in the middle, feel a little light headed or hungry...
I even used to ride my bike over from my office about a mile, then ride home from work about 5 hr later with no effects.
This could be completely wrong, and maybe not PC - but folks (like me) who are over weight seemed to be the ones that were bothered the least.
I highly recommend a big glass of water an hour before donating, and patients...

*my blood vessels run right on top of my nerves, so a metal needle is excruciating- not an issue for blood samples, but won't work for sitting and donating over several minutes. Odd, apparently, but not unheard of.