I usually give a TV show or movie, that includes anime, cartoons, about 15 minutes. If it doesn't get my attention, I move on. It has to grab me so I can keep going. I know some are slow to start but I want to be at the edge of my seat & literally glued to the TV at the start.
I don't have any universal answer and I would usually wouldn't drop an anime. I would probably try to complete it as soon as I can, if it's not too interesting for me