Maximum cabin pressure differential differs with different types of airplanes. For example, the Cessna P210 max diff is 3.35 PSI, so if you're flying above 20,000 feet, the cabin is going to be above 10,000 feet. From what I've seen, airliners have max diff pressures of 7.5 to 9.4 PSI. Rule of thumb is that the cabin pressure is at about 8,000 feet at cruising altitudes. (Which was below the elevation of my house when I lived in Colorado.)
