They say

"About 1-3 psi per month is normal for air loss, but you still should check inflation rates often, and re-inflate your tires. Neglecting tire inflation for 6 months may take out 6-18 psi, and with the most frequent recommended rate being 30-35 psi, this is a big loss."

Others say: "On average, tires filled with air lose about 1.5 psi every month, whereas tires filled with nitrogen will lose that amount in about 3-4 months."