Desk posture does not fail in a single afternoon. It drifts, a few millimetres a week, as the muscles that hold your shoulders back get quietly outvoted by the ones that pull them forward: the chest, the front of the shoulders, the muscles at the base of the skull working overtime to keep your eyes level with a screen that has slid slightly too low or slightly too close.
A fortnightly or monthly session resets the pattern, but two or four weeks between appointments is enough time for a desk job to undo a meaningful part of that work before the next one. This routine will not replace a session. What it does is hold the ground a session gains, so the gap between appointments does the least damage possible.