Depends on the source.
If I'm doing an entire disc, I'll burn it DAO @ 32x (the laser never shuts off).
TAO I'll go as high as 8x but no faster.
e6400 Core 2 Duo @ 2.13ghz
GeForce 7600GT 256mb PCI-e
2gb DDR2 667mhz Patriot ram 1.8v in d/c
Gigabyte GA-965P-DS3 F10 BIOS
WD Caviar SE16 250GB SATA300 7200RPM 16MB Buffer
Samsung SATA2 80gb 7200rpm
Samsung SH-S182D 18x DVD burner