To figure out your interval, use this equation:
(Total Timelapse Duration in minutes x 2)/Desired Length of Shot in seconds = Interval in Seconds
So if you were going to set your camera up for 60 minutes and you wanted a 5 second finished shot, your interval would be 24 seconds.
To stitch together your photos, use Quicktime. Put all your photos in a folder. In QT select File>Open Image Sequence... and select the first shot in your folder. Select your frame rate (usually 29.97) and QT will do all the work.