Question

-(void) play
{
    CMMotionManager *motionManager = [[CMMotionManager alloc] init];
    [motionManager startDeviceMotionUpdates];

    BOOL timeReached = NO;

    while(!self.stopButtonPressed)
    {
        if(motionManager.deviceMotion.userAcceleration.y >= ... && motionManager.deviceMotion.userAcceleration.y <= ...)
        {
             //start timer
        }
        while(motionManager.deviceMotion.userAcceleration.y >= ... && motionManager.deviceMotion.userAcceleration.y <= ... && !timeReached)
        {
            if(//check timer & if timer is >=300ms)
            {
              timeReached = YES;
              NSLog(@"acceleration on Y-axis stayed between ... & .. for at least 300ms");
            }
        }
    }
}

Answer 1

It seems like you’re going about this the wrong way; instead of running a clock, you should be setting accelerometerUpdateInterval to whatever you’re looking for and using startAccelerometerUpdatesToQueue:withHandler: to receive the data.

From the Apple documentation:

Handing Motion Updates at Specified Intervals

To receive motion data
at specific intervals, the application calls a “start” method that
takes an operation queue (instance of NSOperationQueue) and a block
handler of a specific type for processing those updates. The motion
data is passed into the block handler. The frequency of updates is
determined by the value of an “interval” property.

Accelerometer. Set the accelerometerUpdateInterval property to specify
an update interval. Call the
startAccelerometerUpdatesToQueue:withHandler: method, passing in a
block of type CMAccelerometerHandler. Accelerometer data is passed
into the block as CMAccelerometerData objects.

Once you’ve set everything up, let it run. When you’ve received a completion notice from the block look at the data you’ve received.