Way back when - when I was doing a lot of walking - I noticed a weight loss platue around 1 month. I lost a little afterwards, but not much. But my legs sure looked better by the end of that summer.
From what I learned in my weight lifting class last year, you need to change what you are doing. If you do a lot of walking, then do some biking or swimming instead. Rotate what you do every 4-6 weeks or when you find yourself platueing. Everyone is different. When you start a new activity, you body has to adjust to the change.