How is the video length

How long can one single video record?

The capacity of all videos in the action camera should be less than 4GB after the SD card formatting to FAT32. You have to format the SD card before using it.
1) For action cameras of all models except for V50 Elite if the loop recording function is not turned on, the videos recorded with the resolution of 4K or 2.7K, 1080P, and 720P will be saved every 9 minutes, 25 minutes, and 37 minutes respectively till the SD card is full.
2) If the loop recording function turns on, you can record videos for one, three, and five minutes with the resolution of 4K or 2.7K (It will vary from different cameras). You can also continuously record videos that last for 10 minutes with a resolution of 1080P. New videos will cover the existing ones when the SD card is full.
You can't apply the loop recording function to record videos with the resolution of 4K or 2.7K if your action camera is EK7000, EK7000Plus, EK7000 Pro, and EK5000. But you can do it with the resolution of 1080P or 720P and 10 minutes is one video. New videos will also cover the existed ones when the SD card is full. The loop recording function can be applied in action cameras of other models to record videos that last for one, three, and five minutes with a resolution of 4K or 2.7K.
3) You can continuously record videos that last for 136 minutes with a 64GB SD card without applying the loop recording function.

Is there any time interval while using loop recording?

There won't be any interval in the loop recording if you do not turn on the loop recording function. But there will be an interval of 2-3 seconds if you do not turn on the function.