Most of them say once every 30 days when you get the blue box. Remember though that you can use different banners on different pages for each company and still get all the points. Did that make sense?
For example Tupperware usually has 5-8 different banners that show up on different pages. You can earn 800 points on each banner! Last month I earned 4800 points just from Tupperware! You can also use the ones that come through email or the box that pops up when you first sign on. Just make sure you keep track of which banners you have already used.