How many bags of kitty litter do you get thru a month?

[lljkk]

I need it referenced by volume or weight.
Trying to figure out how many to get to start (2 cats indoors for first 7 weeks or so of ownership), and whether it's worthwhile getting a large bulk order, especially given they may go outside for all their "business" starting in about 8 weeks.
TIA.

[SecretSquirrels]

I use the wooden pellets and I have used just under two large 30 ltre sacks since we got kitten at 9 weeks.
He has a very large covered litter tray.
He is now 20 weeks but won't be going outside until he is neutered ilater this month. Even then I don't know how long it will take to teach him to do it outdoors. I presume he will only be outside under supervision at first.