A popular misconception is that probiotic microbes colonise the gut on a durable/permanent basis. This is simply not true. Probiotic microbes tend to persist in the gut for several days to a week. An exception to this can be bifidobacteria given in early infancy.
There is some limited evidence that these may become part of the gut flora in the infant on a longer-term basis. But for the overwhelming majority of people the persistence of supplemented probiotic microbes in the gut extends out to a maximum of about a week.