The PhoneBuff team used robotic devices to perform tests on two iPhone XS models, one with dark mode enabled and one not. Testing included watching videos on YouTube, using Twitter, navigating Google Maps and chatting on the Messenger app.
At the end of the test the iPhone XS without dark mode enabled was dead, while the other dark mode still had 30% battery life. This result should be enough to make you use dark mode especially if you use the phone intensely.
Keep in mind that these are not official tests. Also the phones were at 200nits brightness and you can get different results according to the screen brightness levels. Also testing included applications with dark mode. The latter gives a much higher impact on OLED models than LCDs since OLED pixels turn off individually when the view is black.