pH Pen Calibration Frequency?

Calibrate your meters monthly is whats generally recommended. I don't personally do that because i am lazy. I usually calibrate before the start of a new grow and then just ride that through. After awhile you will get to know where your water sits and what you need to add to get the pH in range. You'll get familiar with the routine and will spot something out of wack if it's severe. Slight changes like 0.2 are hard if not impossible to detect. That may not seem like much, but with a pH of 5.8 when it's actually 5.6 because your pen didn't read accurate can cause an issue in nutrient availability. Rinse off your pH meter after use with tap water. Don't touch the glass bulb, ever. Shake the tap water off or wipe the plastic exterior, then place the cap back on the meter. The glass bulb can not dry out so get yourself some storage solution to pour into the cap. Some people use tap water which i don't recommend. Probe storage solution is reasonably cheap, and is designed specifically for 1 purpose and will last you years.