Well, there is no strict minimum, but with regards to strength training, I believe that the minimum for noticeable results is 2-3 times per week, at least 2-3 months in a streak. Anything below that is rather inefficient because you need some consistency in order to get in a good strength training form and effectively reap the benefits.

Thank you for sharing the example about the benefits of gym training for yoga. Yes, I agree that for ordinary people it is the best to engage in different types of workouts and change them every couple of months or years. Cross-training is often practiced even by professional athletes. : )