The amount of audio you can get on a CD depends on the format used to store it.
When you copy LPs or music cassettes (or indeed official CDs) to computer then you can use a number of different formats, such as WAV, MP3, WMA etc.
And even within those formats you can use different bit rates to make the quality good, medium or low.
The format used by "official" audio CDs can fit about 80 minutes onto a CD.
But it you converted them to MP3 you could probably get hundreds of minutes onto a CD.
So a lot of it depends on what format you will use to store these cassetes on CD.