Whenever it bothers you. It is hard to answer this question with a number.
Some pianos are very unstable, especially pianos with new strings.
It takes many tunings before new strings become stable. Some pianos are in environments where there is frequent change in temperature and humidity. Some people are more sensitive to pitch irregularities than others. I never recommend or reject a piano because of its name. It is better to judge each individual piano on its own merits.