This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
It depends on how much data you're going to burn, and how fast your drive is.
Burning 650MB of data takes about 74 minutes at 1x, 37 minutes at 2x, and
19 minutes at 4x, but you have to add a minute or two for "finalizing"
the disc. Remember that single speed for CD-ROMs is 150KB/sec, double
speed is 300KB/sec, and so on.
If you have half the data, it will finish in (about) half the time. If you
record the same thing twice as fast, it will finish in (about) half the time.
Most CD recording speeds are linear, i.e. recording at 12x is twice as fast
as recording at 6x. If the drive uses a PCAV mechanism (see section (5-22))
the speed varies depending on which part of the disc you're recording.
If a "20x" drive uses PCAV to get 12x at the start of the disc and 20x
near the outside, you know that burning 60 minutes of audio will take
somewhere between about 5 minutes and about 3 minutes.