Kefir stays good for up to a week past its date. Opening the container doesn't change much in that matter. Its taste becomes stronger over time, and when its around a week past that date, it will often be too strong to drink.

How long do kefir grains last?

Putting the Grains on Pause

More milk for longer breaks is best, to keep the grains well fed. Put a tight lid on the container and place it in the refrigerator. The milk kefir grains should be safe and healthy for up to 3 weeks.

Do kefir grains go bad?

Mold can change the quality of the whole kefir, so it's best to throw it away in that case. Also, the separation of kefir grains from the milk and a change in taste or smell is a good sign of kefir going bad. You can drink kefir even after its best-before date if you like the strong sour taste.