I keep 2 pens calibrated and when I start seeing different readings I will check both. Probably 3 times per grow. I got bit once because I was using one pen and by the time I realized I was off I was having issues. If you think about the cost of the pens in comparison to the time invested in the grow it makes sense to me to double check.
For me its when something doesnt seem quite right with the plants, PH is the #1 usual suspect so I start there. I recently found out the Blue Labs auto dosier probes dont last forever, but 3 years and $45 for the replacement seems reasonable.