Files automatically compress after 14 days if not touched. That's the default setting. You can certainly increase or decrease it. This assumes that Compression is enabled and active, and you're not doing anything funky like setting don't compress flags. The compression engine launches a process at Midnight (default) to scan for any files that meet the criteria, then compresses them if they do.
One thing you could do, with a space dilemna, is make sure that files are not decompressing if they are already compressed. Do this with the "Set Convert Compressed to Uncompressed Option = 0". This will leave them compressed.
Also note that any file over 300MB will not be compressed at all. That's a hardcoded setting that you cannot do anything about.
marvin