From my personal experience with US Galaxy S6 and Galaxy S8 I could never record more than 8 minutes of 4k without the phone giving me a heat warning and stopping recording and on top of that won't let me record giving me a "phone is too hot wait for it to cool down" message.
Could be cause I live in Florida maybe the heat/humidity has something to do with it.
Trust me, I wouldn't have asked online about the limit if I wasn't excited about finally recording for as long as I wanted in 4k30fps it was a feature I've been wanting because of the hassle I've always went thru.
Do you know annoying it is to record a miniature golf video of a few friends and having to wait at a hole for the phone to cool down before I could start recording again? cause I literally went thru that

