Actually, with ConvertXToDVD it varies, depending on the codec and compression used in the source file. Sometimes, you get a DVD when converted that won't fit on a DVD-5 even though the contents are under an hour. And sometimes you can get over 2 hours of footage on a DVD-5. Just had one recently where I got 2 hours and 19 minutes of footage on a DVD-5. However, that video was mostly just an audio stream and 2 hours and 19 minutes of a still image in the video. So, that wouldn't take too much space to begin with.
It's not the time length that matters, it's the video compression that matters. For instance, I have a zombie movie collection where they put 4 movies on each DVD-9. They compressed the hell out of them but managed to get just under 8 hours of footage on the disc.
Unless you are really that concerned about the video quality, I wouldn't worry that your source file converted contents fit on a DVD-5. MPEG 2 DVD Video is generally compressed anyway. However, you rarely see the results of the compression. And if you have an upscaling DVD player, you won't notice it at all, generally.
You can do what I do. I often times get a VIDEO_TS that fits on a DVD-5 but I put the source file in the root directory of the DVD as a backup. This can result in a DVD-9, but not always. I like to have a backup of the file on the disc, just in case.