Another chime in to say 3-5psi per day, "bottoming out" at ~75psi. This is for 700x25 road tires. Starting lower, they lose less. My 700x35 drop just 1-2psi per day starting from 60psi, and don't generally get below 50psi at all.
But I air up before every ride, so it's a non-issue. Unless a tire is dropping +15psi overnight, it's not a problem.
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