Don't take my word for it, but as I remember it, the consensus is that it's about a 3:1 ratio.
Meaning, if you train hard for a month, then do absolutely nothing for 3 months, you'll be back to where you were at the start of the first month. That's for all the work you've done to be completely reversed, though. You'll start seeing decreases in performance quite quickly. Also, realistically..the time periods involved would be larger than the simplified example I've given.
However, if you do less than you would normally rather than nothing at all, you can probably maintain where you're at.
I'm curious, why do you ask?