You can specify the time frame for Events in your photo library.
To set the time frame for your Events:
Choose iPhoto > Preferences, and then click General. (If you’re in full-screen view, move your pointer to the top of the screen to make the menu bar appear.)
Choose an option from the “Autosplit into Events” pop-up menu.
You can choose one Event per day or per week, or specify two-hour or eight-hour time frames. The hour-based options are useful if you photograph more than one Event in a single day.
As you import more photos to your library, they’re organized into Events using this new time frame. Changing the time frame doesn’t affect the Events already in your library.