My opks often go darker and lighter until I ovulate then it's bam really dark test line. Mine never have a beautiful fade in and out. Cd19 is too early to stop. I don't know how long your cycles are but I have long cycles around 35-37 days so I don't ovulate till cd26 and I have an 11 day luteal phase. It took me a while before I figured that out so I would have missed it if I gave up at cd19. Keep testing twice a day in cheapies, I heard afternoon testing is good for opks. And I would test up until AF or BFP until I knew my cycle and the other signs that can help back up that opk like cm, ovulation cramping or I always get sensitive breasts for 1 day when I'm ovulating.
Keep testing! Xx