Dual layer DVDs - SP mode is 4 hours?
 

from e-mail...

Although DVDs are advertised as being measured in "hours", it's mostly done to help transition people from VCRs to digital discs. A VHS tape was hard-wired to record certain pre-determined lengths, be it SP mode (2 hours), LP mode (4 hours), or EP / SLP modes (6 hours) -- on standard T120 tapes. Note that T130, T160, T180, T200, etc -- those were different recording times available.

A disc has no such limitations. Video is now nothing more than digital data, and the amount of data that can fit on a disc is solely determined by the bitrate used. DVD recorders have a number of preset "modes" named similar to VCRs -- XP (1 hour), SP (2 hour) -- and the LP, EP and SLP can pretty much mean whatever the DVD recorder manufacturer says it does, there is not any real consistency here. There are also some gooyf modes such as SP+, SPP, etc, and each length is determined by that manufacturer. In each case, refer to the machine's manual as to how many minutes or hours fit the mode. Some machines ever have "flexible" or "FR" (flex recording) modes, that let you specify how much you want on a disc in preset intervals (often 5 or 10 minutes).

If you're using a computer, there really isn't a "mode" or time limit/max.

If you're using a DVD recorder, then know that not all machines can use DVD+R DL media. And of those that can, it's usually pretty poorly implemented. Burning DVD+R DL media (or DVD-R DL media, for that matter) is not really suggested. For a DVD recorder, for the best experience, I suggest you stay with normal DVD-R and/or DVD+R for archiving. Only use DVD-RAM, DVD+RW or DVD-RW for temporary recordings.

Even if you're able to succesfully record to a DVD+R DL in a DVD recorder, odds are the the layer break won't be well understood by a normal DVD player, so you could only watch the disc in the recorder. Not a good scenario. Layer breaks are tricky animals, and recorders just don't handle it all that well. An infamous problem.