The only slight problem with kefir (both water and milk) is that there is no off switch. So what do I do when I have enough kefir? This is the question I am asked most often.
The simplest solution is to prepare your grains as if you were brewing with them (i.e. put them in a clean jar with the usual amount of milk) and put them in the fridge. They can be left to sit in the fridge for between 5-7 days without the need to change the milk again. Keep an eye on them and if you see complete separation, you need to change the milk (otherwise you risk them running out of nutrients). When storing kefir in the fridge I recommend covering it with cling film or similar rather than a cloth as you do not want to risk your kefir being contaminated in the fridge (I personally don't like brewing kefir with the lid sealed on Kilner style jars as there is always a risk of explosion if the CO2 levels build up in the jar, this is rare but very messy when it happens!) The kefir drink that is produced in the fridge is safe to drink and can be bottled and consumed as normal.
I would not recommend always keeping your kefir in the fridge as the cold tends to inhibit the micro flora in the grains and therefore can eventually damage the grains. It is best if you always ensure that for every seven days you keep your brew at room temperature for at least 48 hours.