Not sure if it's the same for different types of hernia but I've had two epigastric hernias (which are located in between the belly button and breast bone and are due to a weakness in the connnective tissue of my abdominal wall) and neither of them showed up on the scans. But I was told that they don't always show up on scans and that they go on symptoms and examination. Having said that my surgeon was convinced I didn't have a second hernia (the internal stitching from my first hernia was diguising them being able to feel the second hernia)...right up until the point he opened me up and found the second hernia! I know I have a third one but the next time would require more extensive surgery and as I'm still recovering from having my gall bladder out I'm delaying it as long as possible. Mine only troubled me when long distance running or doing sit ups and press ups - neither of which I am currently doing.
Waiting times depend on your local situation. Non emergency surgery tends to be 4-6 weeks round here.