I use 60mm Big Apples inflated once a month to something over 2 bar. The tyres actually feel hard to the hand but are very comfortable. I use a gauge to determine whether they've lost air. In my experience the amount or harshness of riding has little to do with how much air is lost. I suspect the change over a month has more to do with barometric pressure and temperature changes. As far as I can tell, the normal Schwalbe tube and the thinner, lighter Ultralite, which I use interchangeably, lose air at the same rate. So I have data points going back ten years (when I switched over to the Big Apples) but no really helpful conclusions. Perhaps that's how it's supposed to be on an almost Zero-Service Bike.