使用手机摄像头使用getUserMedia录制具有设定持续时间的视频

I want this through web browser not native app. User allow the phone camera to access camera, then start recording video. Every 3 seconds (for example) capture what the user records(maybe into an Blob). Repeat until user want to stop. So there is a limit on duration. I know you can use getUserMedia for the function, but I dont know hot to set duration for the video camera recording.I want this through web browser not native app.