You can fit about 70-80 minutes of video on a DVD disc.
It doesn't matter how large your file is now. The program will convert it to VOB files when you select the option to burn a DVD and, in this format, a 4.7 gig DVD can hold about 70-80 minutes.
I would not ask your Best Buy techie for any further advice, BTW. He apparently doesn't understand how DVDs are created.