1. For time appearance : Duration
There two cases : Toast.LENGTH_SHORT
(display in short period of time) or Toast.LENGTH_LONG
(display in long period of time).
You cannot define a time appearance for toasts
2. Toast gravity : (display in the middle of screen) :
You can position the Toast as we desire using a method named setGravity() which has got three parameters: gravity , x-offset , y-offset.
toast.setGravity(Gravity.CENTER, 0, 0); // toast in the center of screen
EDIT : For your case you can use :
Toast toast= Toast.makeText(getApplicationContext(), getString(R.string.positive), Toast.LENGTH_LONG);
toast.setGravity(Gravity.CENTER, 0, 0);
toast.show();