How much gb will over 1,000 photos take up on the iPhone 5?

I have 7.4 gb left.

There's no way to answer this as it dends on the file size of each photo
a single photo can range from 10mb to 2gb and the file size depends on the number of megapixes your camera is and what quality you set it at along with other camera settings, if its a stndard or panoramic image etc.

Depends on several factors, like camera used being one of the major ones.