Co2 ppm

So I’ve just set up my Sealed room I’m about to flip to flower in next day or 2 I’ve set up the target ppm at 1100. I just want to ascertain how sealed my room is and how much the ppm should drop in relation to time. In other people’s experience how much does the ppm on their monitor drop per minute. Or how long does it take to drop 5 ppm? And how long does your tank last in relation to that drop rate? I have a 12x8x8 ft room and I have 20kg of co2 . Much appreciated