In principle, there is no generally applicable consumption recommendation for milk kefir. But as with all other fermented foods, you should approach the pleasure slowly. Recommendations from relevant specialist literature advise taking one glass with meals or one glass in the evening.
Especially if you've never had kefir before, start with a small glass and give your body time to get used to the living microorganisms.
There are many testimonials from people who drink half a litre a day and have only positive things to say. Milk kefir is also popular during Lent, as it is said to curb the appetite.