pH: how often using same testing liquid / pH changes in larger cans over days?

Hey Mate, I used to mix my own calibration mixtures, but they are ony good for 1 weeks, and the powder for brewing the solution is too expensive. But Mikwaukee is producing cheap calibration-solutions, which they sell in a small bag you have to ripp open. One pH4, the other pH7... open up, calibrate your pH meter and throw the bags away. Costs in my local growshop are €1,50. I calibrate my pH meter every 2 weeks. But I have a semi prof device and I don't know how often you should check the calibration with other brands. Best of luck and happy trails for the rest of the way 🤞 Servus OimRausch