I use the Oko. Zooplus do it for like £22 for 40l, it does come in a massive 40l bag though. I got mine and they came with grit style clumping litter. After about 4 days they were switched onto this. Pets at Home sell smaller bags in case you want to check it out first. I understand that the gold may be better with tracking and I think I may try that next. It does track but then I'd rather have wood bits tracked than small stones. I have a mat, but not a rubbery mat to stop tracking. I would say the furthest litter has gotten though is only about 4m. I have 2 boys and they share a tray. It gets nicely filled up about 4-5 inches deep. It clumps amazingly well. Best thing though is its flushable. You need to kinda wait for it to fall apart in the loo, and then flush each lump separately but it makes a huge difference to putting it in a bin when my bins are communal bins a long hike away. I have a handheld dyson anyway as they're mucky pups with the food too, so I use that for any small bits tracked about.

Anyway, I fill it up, maybe a little top up every few days. I do a total clean out weekly. I had dabbled with every 10 days but as it's 2 using it, I switched quickly back to weekly. In that week I get through 8litres. I would imagine with 1 cat probably about 7litres.