iCloud: How long are photos stored?
Photos stored in iClouds photo stream are only stored for up to 30 days and up to 1000 photos (whichever you reach first). You should save any photos you want to keep to your photo roll or iPhoto within the 30 days allocated.
For this and more information about iCloud check out the iCloud FAQ
In November 2013, Apple removed the limits on photo storage and other than hourly, daily and monthly upload limits - you can store as many photos in the cloud as you care to.
https://apple.stackexchange.com/a/108911/5472
The throttle points are currently:
- 25,000 photos per month
- 10,000 photos per day
- 1,000 photos per hour
From Apple.com - Photo Stream
iCloud manages your Photo Stream efficiently so you don’t run out of storage space on your iPhone, iPad, or iPod touch. If you have Photo Stream enabled on your iOS device, every single photo you take appears in a special Photo Stream album that holds your last 1000 photos. You can delete any photos you don’t want from the Photo Stream.1 To touch up a photo or keep a favorite shot permanently, simply save it to your Camera Roll. iCloud stores new photos for 30 days, so you have plenty of time to connect your iOS device to Wi-Fi and make sure you always have your most recent shots handy.
So your photos will get deleted if you reach the 1000 photos limit or the 30 days limit. It's important to note that photos stored in iCloud by Photo Stream doesn't count towards your storage limit.