IDVD is limited by the duration of your movie and nothing else.
A single layer DVD is good for about two hours. A dual layer DVD is good for about three hours and 45 minutes.
You do need a certain amount of free space to create DVDs.
To burn your iDVD project to a single-layer DVD disc, you should have 8 to 10 gigabytes (GB) of free disk space on your hard disk, depending on the size of your project. For double-layer discs, you need 15 to 20 GB of free disk space. To check how much disk space you have, Control-click the hard disk icon on your desktop and choose Get Info. Then look next to Available to see how much space you have.
The encoded DVD files remain on your hard disk inside your project’s folder. If you do not want to save the encoded files after you burn your project to a disc, you can delete them.