The answer you will find universally for this questions is: "It depends." ;-)
Specifically, it depends on the "compressibility" of the source you are compressing.
Some things compress well, eg. text
Some things don't compress at all (or so little as to be meaningless): eg. compressed files
Once you have a reasonably good idea about what you intend to store in the partition in a compressed format, you will have to do some trial and error to determine how well it compresses and whether or not it will fit.
I don't think there's any other way to determine an answer to your question, but if there is, I'd be interested to know as well. :)