Hi kat,
I have had the same for a year now and found bag changes depend on what you are up to on a daily basis.
When I was recovering at home, 3 months of being really ill, I once managed 6 days on the same bag, stoma nurse said it was a record lol.
Since going back to work I now change daily as I don't want to risk those leaky moments. They still happen once in a while btw! Also you get to know how the bag feels, I can usually tell if its gonna cause an issue. Itchy or burning sensation on skin, badly fitted bag or ever changing stoma size all contribute to an ostomy wearing disaster!
My advice is keep getting that prescription and keep getting enough to cover you. I get two months worth at a time, if I have bits left over then it a bonus!