Burning a DVD is easy for you to arrange, but it's a lot of work for a computer. The compression required and the encoding into the DVD format is what scientists call computationally complex, meaning it requires a lot of math. Your Mac's processor (G4 is good, G5 is better) and clock speed (the megahertz or gigahertz number that is bantered about) are the important elements. The faster the clock speed and larger the processor, the more quickly your DVD will burn.
Just like burning a CD in iTunes, all you really need to do is click the Burn button. It will open up to reveal the "real" Burn button, with a warning symbol.
Click the Burn button, and iDVD will coach you through the rest of the process. Insert a blank DVD in your SuperDrive, and that's it.
How long iDVD takes to burn a disc is highly variable. A rule of thumb is twice the length of time required to play the discmaybe an hour or two.
Some advanced settings can augment the speed and quality of the disc you burn, but the defaults are generally appropriate.
If you're going to make multiple copies of the disc, now is the time to do it (when iDVD prompts you). It'll take much less time to do it now than later.
When the recording is done, iDVD will eject the disc for you.