If you have so many plants that even after half hour of watering them, you are still not done, then you are quite possibly insane.
Dear Reader, if you think this describes you then you should see a Mental Health Care Professional as soon as possible. (Feel free to give said professional a green gift).
If you think it will be hard to reduce watering time, I am sure there are other insane people dying to get some of your plants. I am also certain that there are people inside your very own house who would willingly help you reduce your collection in some way…or even tie you down so that you cannot water these plants, thus allowing nature to take its course.
Imagine the freedom! 15 more minutes a day is bliss!