Hi Nell,
Sorry for taking so long to reply - I've been away for a few days.
They are standard size 2 ply couch roll, so it depends on whether you split the roll or not - I was taught to split the roll to be more economical, but I prefer to use it as it is as I find it's too thin and tears too easily if you split it down to 1 ply.
I only work part timea couple of evenings a week and so a roll lasts me absolutely ages - sorry I can't be more specific! I probably use a roll every few months....