Hi Mals!
Hmmmm........ A good question!
Let me see! On an average its once a year or so.
I hate baggy undergarments!

And I hate it even more, if its slightly fading or discoloring!
I buy about 5 panties, and 3 bras, and I am good to go, with rotating these with the existing ones!
I use panty liners everyday too!
It will be interesting to follow this thread, I will be watching it everyday!!