The length of the fotage should not affect your quality. What affects the quality is first how good is raw footage, secondly what you are doing to it and lastly how you are compressing it.
Okay, I will assume that you are doing point 1 and 2 perfectly, everything is set up and it looks fantastic BEFORE export or compression, right? I am also assuming that you want to compress for DVD and not for web, iPod, apple TV ...etc.
For DVD SP4 you can even use a Non-selfcontained FCP project file (as long as your media files are available on the same or linked drive) to do your compression. Please use in your settings for compression DVD MPEG-2 SD (or HD, if that is what you are shooting) and also check the options of Mode, Bitrate and field order. There are some guidelines there, if you google them, but normally the default settings do work quite well. in iDVD is the same.
The quality should not really suffer from the length of your footage. It is just a longer workload for your machine, not a harder one!