Today I donated blood for the 14th time (I think it's actually more than that but I did have a long break). Sometimes my blood can be a bit temperamental and on several occasions they've had to try both arms to get it to work, and on one occasion it was so slow they had to stop my donation :(. Usually though when it does want to work it works quickly, today I decided to time my donation and from the moment the needle went in to the moment the red light started flashing was approximately 4 minutes, pretty quick even for me. So the actual donation can be pretty quick (no longer than 20mins), if you make an appointment then the wait is not too long and you get a cup of tea and a biscuit for your trouble, today's appointment took up less than an hour for me. So what I guess I am trying to say is if you can give blood then I would wholly recommend doing it, take an hour out of your day once every 4 months and do your bit.
You never know who may need that blood to save their life. :)