That all depends on kind of data that is going to be compressed. Graphics will compress less than text data will. Binary data will compress less than text data will.
If you are compressing text data only it depends on the value of the data you’ll be compressing. If all the values are the same it will compress well. If the data is very unique within the files then the data will compress less.
Your data could compress down to anywhere from 50 Megs or so to 1 Gig (almost no compression).