I have been asked many times by customers about how to know how much music is needed for a DVD slide show. Unfortunately the answer is it depends, but there are certainly some rules of thumb to help you get started.
First, you need to determine the number of slides you'll be using -- that includes all your photos and title slides. Each "slide" will be shown for about 5-8 seconds in the final project, but this is only an estimate. You'll want to multiply the # of slides by the number of seconds and that will tell you how many seconds of music you'll need.
As you are likely to have more than 1 song in a given slide show, you should ensure the song's length will cover all the photos you want to include in that section.