Error while capturing video for a pre-defined time period - opencv

Question

This program captures video until I press Esc. But I need to modify this program and capture video for 30s.
After recording the video it plays very fast and the video length reduces. I tried to add waitKey at the end, but it still doesn't work. After recording the video length should be the same. How can I do that? Any suggestions?
int main( int argc, const char** argv )
{
using namespace std;
using namespace cv;
VideoCapture cap(0);
while(!(cap.isOpened() && cap.grab()))
{
cout << "Camera not ready" << endl;
}
VideoWriter Writer("D:/MyVideo.avi", CV_FOURCC('P','I','M','1'),20,Size(640,480), true);
while (waitKey(30)!= 27)
{
Mat frame;
cap >> frame;
Writer.write(frame);
imshow("D:/MyVideo", frame);
}
}

Answer 1

int64 t0 = cv::getTickCount();
while (waitKey(30)!= 27)
{
Mat frame;
cap >> frame; // read a new frame from video
Writer.write(frame); //writer the frame into the file
double t = (cv::getTickCount() - t0) / cv::getTickFrequency();
if (t > 30)
break;
...
// now, we need to adjust to the desired framerate of 20fps,
// so we need to sleep for 1000/20 = 50 milliseconds
// either have a window, and use waitKey():
// imshow("lalala", frame);
// waitKey(50);
// or just sleep(), unfortunately system dependant ;(
// win:
// Sleep(50);
// linux, etc.:
// usleep(50);
}