This is pretty common on all cars I've worked on. The leak is so slow that you will never see it in 30 minutes. You will need days, to see anything significant. The leak is just so small that it takes several months to get to a level where you need to add some. Next time put a can with dye in it, then look for it next summer. I have had cars that only took a can a year. If it's that low I don't even bother with fixing it. My wife's 96' Lumina would leak enough that I put a can in around March and again around October. We run AC nearly year around here. We had that car for about 4 years and I never did anything else to it AC wise.
