'Sometimes I can meditate for half an hour, with a sort-of-clear mind, and feel relaxed afterwards.'
This sounds like the ADD is not interfering with the meditation time, many would give their right arm to have this!
As Giles says, meditation periods are very variable, even when one is very experienced.
Sometimes 'trying to focus on one thing' just increases our tension. Just allowing the images/thoughts/feelings to be there and be aware of them coming up and then fading away all by themselves is a great meditation and will allow you to relax. You only need to come back to your one thing when you realise you have caught one of these images etc and are running with it, so you can then let it go.
If you are not feeling good at the end of your meditation period, take a few minutes just to relax. Breathe very slightly deeper than normal and then relax on the out breath. You can just do a 'global' relax or go through different parts separately. Deliberately think kind, peaceful and accepting thoughts about yourself.
peace and joy

H