There is no length limit for photo blogs. They are based on KeepandShare's powerful document editor which can accommodate hundreds of pages of text and photos. You can insert as many photos into your photo blog as you like.
However, since your friends will be visiting and viewing your photo blogs and they must download the photo blog to view it, we suggest you keep your photo blogs to perhaps 20-30 photos each. So if you are documenting a longer story, such as a major vacation, create several photo blogs to break up your story, such as "Ireland Trip 1 - Dublin" and "Ireland Trip 2 - West Coast".