The amount of time taken to write a disc depends upon the speed of the recorder, the media, the writing method used by the recorder and the amount of information required to be written. In theory, to write a full 4.7GB disc, it would take:

25 min. at 2.4x speed
15 min. at 4x speed
8min at 8x speed
5 min. at 16x speed

NOTE: The higher-speed drives (such as 4X for DVD-R or 2X for DVD-RW) won't work at their top speed without the right media. In other words, a 4X DVD-R drive won't be able to write at 4X if you use a (less expensive) 1X or 2X DVD-R disc.