Hi Broads,
My experience was about 2 minutes to put the drops in my eyes, 20 minutes wait for the drops to make your pupils dilate, then 5 minutes for a visual check and the photos to be taken of your retina. As the photos are taken, you will see flashes of bright light, but it is not normally uncomfortable. So about 30 minutes if they are running on time and you don't have to wait.
The eye drops can cause slight stinging, and your vision may become blurred about 15 minutes after the screening ends. The blurring can last between 2-6 hours, depending on what sort of eye drops are used, and will most likely affect your ability to drive, so if you are driving there, get someone to drive you back home. You may also find that everything looks very bright during this time, and bright sunlight can be quite painful (if we get any!), so you take sunglasses with you.