The number of minutes of music or megabytes of data which will fit on the disc is printed on every blank CD. You can check the capacity of your disc using the Medium-Info item on Nero's Recorder menu. When you overburn a blank CD, the size of the disc is ignored by Nero and data is written to the disc beyond the specified capacity. It is clear that ignoring the capacity of the disc entails a certain amount of risk.
This applies both to 74 and 80 minute CDs. In addition the CD standard contains a restriction which states that there are no CDs available with a capacity larger than 80 minutes. This means that all CDs are seen by Nero as 80 minute CDs even if they are sold by the manufacturer as, for example, 99 minute CDs. You can only write 99 minutes of audio to the CD by overburning.