All backup files are compressed and encrypted before uploading to the Backup Server, the average compression ratio for text-based file is around 4:1. However, no further compression can be made on files that are already in compressed format (e.g. JPG, ZIP, PDF). In general, you can assume a 2:1 compression ratio (or 50%) when you are backing up a file set with variety types.
It is difficult to provide any sort of guidelines specifically describing the compression ratio you will receive. This is because there is absolutely no way to determine the compression ratio of a file without actually compressing the data.