I stopped lifing weights two years ago when my shoulder joints started protesting.
But before that... I only increased weight on the advice of the trainers at the gym, and they did annual assessments. More often, they'd switch machines or suggest more reps instead of increasing weight...
I can usually tell when it is time to increase the weights. But the last two times a friend called out, get more weight, sheesh and you know she was right. Hugs
Great question and I can't wait to see how everyone else responds. Typically I will increase once my current weight gets "too" easy. If I can do 3 sets of 15 reps and not really feel it-I know it's time to increase. I don't go by a calendar date (like every so many weeks or months) but more how I feel.