Before you can leave the Red Cross building, there is an obligatory 15 minute wait in a break room where you are offered cookies and juices and coffee.  This wait is to ensure you don't go into shock and allows for your body to adjust to the lower blood volume.  There is a volunteer host in attendance in the break room and they will engage you in conversation if you care too or leave you alone to your own thoughts, your choice.  They will also offer you a beverage upon which they will often write on your beverage cup, your permissible exit time as calculated from your entry time into the break area.

There are lots of delicious snacks (cookies for us cookie monsters ;=) to bolster your system back up with, as well as lots of different magazines to read as you countdown your personal fifteen minutes while lounging in the break room.

All in all, the last fifteen minutes goes really quickly.

Personally, with one exception, I have never felt any discomfort after giving blood.  The one exception was when I had given blood after having had an earlier session of vigorous exercise immediately prior to giving blood.  What happened is I experienced a mild form of shock requiring ingesting some coffee, (for the caffeine) to get my system jump started again.  I wasn't close to fainting, just a little unsteady on my feet.