it depends, for our planets it is commonly known. For Phobos I would like to have acces to the realtime scientific numbers as we are currently approaching the 1/Pi countdown in our perception of time. In this case it is about seconds. latest I can find is abot 50 seconds away, but ofcourse i am curius about how long it will take that phobos will reach that orbital period. Last interesting info on this that is available on the internet, is that ESA found Phobos on a different position as expected, which means her orbital period is closing losing more orbital period that expected by scientists.