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It's towards the bottom of the icon column on the right side of the screen. Drag the pink line along the timeline to indicate how long you want your video to be; the app will stop recording automatically at that point. A countdown will begin (3, 2, 1…). When the countdown ends, TikTok will begin recording immediately. There’s no need to press the recording button, though.  To pause the recording, tap the stop button at the bottom of the screen. To start recording hands-free after pausing, tap the timer icon again.

Summary:
Tap the stopwatch icon. Select when you want your recording to end. Tap Start Countdown.