When using Photo Stream on the iPhone, you have access to the last 1,000 photos taken, or the last 30 days of photos (source). That same rule applies to all iOS devices, and beyond the 30-day window, no photos are actually saved to your device unless you specifically select and save them to your Camera Roll.
What about with a desktop or notebook system (e.g., iMac, MacBook Pro, etc.)? Are the photos only temporarily accessible through iCloud when you see them in iPhoto, or are they actually stored on your computer?
Best Answer
Unlike your iOS device(s) that have a smaller more finite amount of memory, your desktop/notebook systems are more than capable of storing the photos, as they're found in your Photo Stream, they're also automatically imported to your iPhoto Library (same source as the question above):
Take note, however, that there is a difference between your Photo Stream collection and your iPhoto Library. Attempting to interact with any photo being viewed while Photo Stream is selected will prompt you to view the copy in your Library.