Photo Stream will hold only 30 days or up to 1000 photos from your camera rolls on your existing devices. It is a temporary stream that is not intended for long-term storage of photos.
You should be importing all of your photos to iPhoto on your Mac regularly. You can also set up iPhoto to automatically import your Photo Stream monthly, but, again, the Photo Stream itself is a dynamic storage area that will change as you take new photos.
Maybe you could provide us with a little more information about how you have your iPhoto set up, how often you import, etc. in order for us to be able to best help you.
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