How often calibrate Ph pen?

Kinda odd question because im sure it varies on alot of things, but how often should i calibrate my ph pen?
I randomly woke up this morning and had a dream about my ph pen being way off, so just to make sure i got some 7.0 calibration solution and went to town, turns out in the solution it read over 9.0 :/ so it was wayyyyyyy off, but heres the thing i keep it in half a shot glass of reg tap water and i have since the day i got it, so what made it get un-calibrated? its only about 5 month old ph pen...

Also, Im pretty f&%^ing pissed at myself for not doing it sooner, all the water / nute solution i've been giving my plants since i got the pen i assume has been way off :/

any answers/ ideas ?

I calibrate every week and when i do it's never off by more then .1-.2. I use the cheap Miluakee brand one from amazon,cost me like 15 bucks and it still is a decent pen. I also leave mine in a little bit of the ph solution until i use it again. Then everytime i calibrate i use new solution then store the meter in that. hope that made some kind of sense

THE PUMPKINHEAD said:

I calibrate every week and when i do it's never off by more then .1-.2. I use the cheap Miluakee brand one from amazon,cost me like 15 bucks and it still is a decent pen. I also leave mine in a little bit of the ph solution until i use it again. Then everytime i calibrate i use new solution then store the meter in that. hope that made some kind of sense

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yeah we have the same pen, i calibrate everytime before i use it, never off by more than .2.

I do it every week

i leave mine setting in 4.01 cal solution. if its off. I calibrate.

Cheep miluakee here. Haven't calibrated it in 4 months. And just put the cap on for a week. And it drys out everytime. I use it just for a eye ball test. I use the drops to do the real test and pen reads rite. And I'm mean on the pens but working fine so far.

Here's a hint: There is no point calibrating to 4, only to 7. And, you can use distilled water which is 7.0 both for calibration and storage.