So it's been around 13 weeks now, with a one time partial refill, and I think next time the fleece needs changing I will have to change out the wood pellets. After stirring there is a lot of dust and not so much whole pellets anymore.
I have one piggie in a cage which is 135x65cm (the actual inside space she can use). I used roughly 40 liters of pellets, with a one time extra of 10liter after scooping out dust.
I think, without the refill, I will be able to go three months before doing a whole cage clean.
My girl, however, does love to burrow so there are a lot of poops that end up underneath the fleece. She sometimes goes as far as to pull the fleece aside so she can poop and pee on the pellets instead of on the fleece!