How long will DVD movies play before I have to turn the disc over?

At an average bit rate of 4.5Mbs, a single sided DVD disc has the playback capability of 133 minutes of the highest quality audio and video images. That's nearly 92% of all Hollywood titles ever made!

In fact, a dual layer DVD disc can provide up to 4 hours of the highest quality audio and video on a single side of the disc!

http://www.dvdforum.org/tech-faq.htm