The way I make the sponge, you take about a cup of starter (roughly half of what I keep in the fridge) and add a cup each of flour & water, then leave it to sit until bubbles form again (I leave it over night).

I was reading a blog a few weeks ago about drying out your starter to preserve it! In case you can't look after your jar for a while (new baby/holiday/CBF). I think I need to do that!