It can last in the frig for a couple of weeks plus. They just go to sleep and don't actually die . What kills them is heat (50 deg C plus)
Like Knotted says you can use actual yoghurt as a starter. You can even do this every time (ie keep back 20% of the batch to add to "tomorrow's")

My wee pet kefir is doing really well and stabilising nicely the past few days.