Of what order of magnitude shall be the compression ratio of information ?

Answer 1

That depends entirely on the information, the method of compression and whether one is considering only lossless compression.

MP3 and JPEG files (which are not lossless) typically compress from the equivalent WAV or BMP formats with a ratio of 10%, i.e. one tenth of the file size.

If one is considering a database or spreadsheet file with large amounts of empty space, compression may be a lot higher than that.