You can calibrate it as often as you’d like —every run, occasionally, or, as Stuart put it, until you’re satisfied. Just keep in mind that what you’re calibrating is your cadence and stride length for a specific speed. Therefore, rather than focusing solely on constant calibration, it’s more important to maintain consistency in the gait you use during both calibration and your runs.
By the way, when it comes to numbers, my OCD definitely kicks in—but I’d still recommend focusing more on your actual workout than on the data you’re recording. Trying to get a perfect recording with a foot pod can drive you crazy, so don’t let it take over your training!