As a point of reference, my 1.25GB G4 eMac with 1Gig of RAM tends to take roughly 3-4 times the length of the project. So when I create DVDs of my one-hour home videos, it generally takes 3-4 hours from the time I tell it to start until the finished DVD pops out.
If your iMac is a faster G4 or a G5 is should take a shorter amount of time, where as if you have an older, for example, 800MHz G4, you can expect it to take longer.
On my setup, your project would take roughly 20+ minutes Burn time of the DVD doesn't change, so on short projects the burn time of the DVD itself becomes a significant factor. In other words, if the DVD takes 8 minutes to burn, then that time is nothing compared to a three to four hour encoding time for a one hour movie. But if the project is only 5 minutes, then the encoding might be done in 15-20 minutes but the DVD itself will still take 8 minutes to burn.
Patrick