Travel between the cube faces (transform.Rotate)

I want to travel between the faces of the cube, I want to stop the rotation when the y axis is at 90.00f and continue, if the button is pressed again and for when the axis is 180.00f and so continuously. The code below causes the cube to run perpetually, I want to stop, increment a variable with "variable + = 90.00f;" And continue to run if you press button A again. Please, can someone give me an example?