That depends on the speed of your HD and (perhaps) the quality of your CDR burner. In particular the buffer capacity of the burner.
1 MB is a nice minimum. 600KB/sec (4x) is a pretty paltry data transfer rate for most HDs these days, but it must be <sustained> at this rate for the entire process. If you've burned a coaster, check that no other software interrupted the process before you go and accuse your system of inadequacy and to fix this, select the slower burn rate. You'll only be wasting a lot of time and not improving the quality of your CDRs.