Basically, there is no generally applicable consumption recommendation for water 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.
Especially if you have never drunk water kefir before, start with a small glass and give your body time to get used to the living microorganisms.
However, there are also many reports from people who drink a whole litre of kefir every day and simply feel better and fitter. Here, everyone has to find their own measure.