Galaxy S9 has a 5-minute rule on recording 4K 60fps video

While that's the default on the S9, too, you can switch up to 60fps footage, like on the iPhone 8, 8 Plus, or X, with the necessary warnings about resulting huge video files and so on. There is one more limitation that Samsung has placed on the new Galaxy S9 and S9+, though, and it is likely valid for both the Exynos 9810, and the Snapdragon 845 models.

Yes, we are talking about the 5-minute rule. For various reasons, Android manufacturers have been restricting the amount of time you can record at the highest video settings due to memory, processing power, and other tall orders. First it was with 4K 30fps video that Samsung allowed you to only capture five minutes of at a time, and now the same restrictions go to the newfangled 4K 60fps standard. 

Bummer, but if a Samsung goes all hot and bothered during a task, it's better to play it safe, and limit your breathtaking 4K 60fps sesh to five minutes or less, not to mention the amount of internal memory the footage will occupy at that length anyway, even in the high-efficiency video format that the S9 recordings support.

source: Reddit