I am not sure of the answer.

Are they the equivalent of mp3's? If yes, then probably somewhere between 300-400 minutes worth depending on the bit-rate of the particular songs.

Otherwise approximately 79 minutes 50 seconds worth. It's my guess this will be the more likely outcome.

A few ways to check, one is to put the CD into your computer and if it looks like a regular audio CD then that's how it will be. Open the disk on your computer and check the individual file-sizes. Keeping in mind non-compressed audio uses up approximately 10 MB/minute. And mp3's somewhere between 1-3 MB/minute.

I have never used karaoke disks. However many, many years ago Apple did have a software that could create the equivalent of karaoke song files that worked with QuickTime. These were the equivalent of non-compressed audio. AFAIK commercial audio disks that include lyrics and other metadata are non-compressed, similar to a standard CD.

According to Karaoke on Wikipedia, the earliest Karaoke disks were equivalent to VCD's which makes sense, since that is the approach Apple used. So held as many songs as a standard audio-CD. However, CD+G disks are a little different.

According to CD+G seems to be equivalent to a standard audio CD. Sort of a combo between an Audio CD and a CD-ROM.

There are also mp3 karaoke disks but I do not know if these will play in a karaoke machine because they were originally designed for computer use. Perhaps some karaoke machines can play them, I don't know.

All other googling on the answer suggests the answer is 70 minutes for a CD+G disk. Although one site suggested a maximum of 20 songs, this might have been a limitation with their particular cdg software.

With any luck, anybody who does use Karaoke disks will respond to your query to clarify.

I grabbed this from an FAQ of one site:

CDG
Used by professional KJs, this format will display graphics on a screen when used with a karaoke machine. A CDG karaoke disc will play the backing tracks in a normal CD player. (* this suggests same as a standard audio CD for length !)

DVD
If you want to play the music and see the words on your TV but don't have a karaoke machine choose this format.

MP3+G
Used by professional KJs and singers alike. The download contains two files, a MP3 backing track and a graphics file. You can use karaoke software to play the track on your computer or create your own CDG karaoke disc.

MP4
If you want to use your iPod or iPhone to play karaoke tracks choose this format. It will also play on your computer using Media Player or similar software.