Velocities Turtlebot cmd_vel

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Hi, I have a doubt concerning the velocities, especially the angular one. If I want to make Turtlebot spin 90 degrees for a second, for example, what angular vel. should I set? Would it change something if I also set the linear one?

I have this code:

ros::Rate loop_rate(1); //Frecuencia de realización del bucle (10 Hz)

    while (ros::ok())   //Bucle mientras no se reciba "Ctrl+C"
    {
          vel.linear.x = 0;   //velocidad de avance
          vel.angular.z = 1.57;   //velocidad de giro

          vel_pub_.publish(vel);

          loop_rate.sleep();
    }

I guess that with this code, each message arrives each second. How should I modify this code to make the robot spin 90 degrees in one second?