Opinion is divided on this, so I’ll just tell you what I do, and it works well.
Let’s say you baked on Sunday and you have your kilner jar sitting quite happily in the fridge. Keep in mind that what’s in the jar is actually a living thing, it’s not just an ingredient to ignore and use when you want, you’ll need to give it some love. I’ll leave it in there until the following Friday without touching it; it’s a bit like a mistress, give her a load of attention like you did the previous Saturday and Sunday and she’s quite happy for a week, but at the end of that week, she’s going to need some more loving otherwise she’s going to start misbehaving. So at the end of the week, take your leaven and add another 100 grams of water (always room temp), mix thoroughly, then add 120 grams of flour and mix again. This is the leavens equivalent of a really good seeing to, and you can then pop her back in the fridge and she’ll be quite happy for another week assuming of course you’re leaving two weeks before you next bake.
If we assume again that you’ll be baking on a Sunday, refresh it again on the Friday with your 100 g of water, and 120 g of flour, then on the saturday with 200 g of water, and 200 g of flour, but remember when you put the big lot of flour/water in to put the whole lot in a bowl and cover with cling wrap and put somewhere cool overnight, but not the fridge – its a good time to stick your jar in the dishwasher otherwise you’ll be in skanky ho territory, and you’re not a skanky ho, right?
What I’m getting at with the above is the fact that you must refresh your leaven two days prior to baking, then the day before (with the larger amounts of flour and water). In Bourdains book “Kitchen Confidential” the baker, Adam, says “gotta feed the bitch” so I’ve shamelessly stolen that line, though madam isn’t wild about it.
If you’re not going to bake for a few weeks, just do the weekly refresh with the 100g water and 120 g flour – but hold you say, if I don’t bake every couple of weeks, all this refreshment and my mistress is going to turn into a living breathing 6 foot woman and take over my fridge, my house, and my life. No, it’s not, because you’re going to treat it mean to keep it keen and throw away about 75% of it once you realise that you can’t fit your new flour and water into the jar. Don’t ask me about the chemistry behind it, I’ve no idea, nor do I give a toss, but the natural yeast in the leaven needs the gluten in the flour to survive – to eat, if you like.
Now if you really neglect your mistress, it isn’t the end of your bread world. What you’ll see is liquid floating on the top, it’ll have a slight orangey tinge to it. Just pour that off and add your refreshment as normal. If you plan to go on holiday for two weeks, just build up your leaven until the jar is nearly full the day before you go and it’ll be happy until you get back.
If you REALLY neglect it, the yeast will begin to die, and it’ll start to look a bit skanky. If you are such a slattern that you let this happen, you deserve a damn good thrashing, but all is not lost. Just gently clear away the top, and take a teaspoon or two of the putty-like substance in the bottom, start a new jar and add your flour and water – after a few days, your mistress will again be ready for some hot-oven-action.
When it’s in tip-top condition, it should smell really alcoholic, have visible bubbles and look like this