In most of the cases I've heard of it's more like 20% out of 120%. Some managers like it to be very formal, so you have to dedicate a whole day in the week to your 20% work, which is (almost) impossible to do if you are working on a product that has to meet a deadline.
It's hard to work on more than one thing at a time. However, from what I understand, you can accumulate your 20% time so that you can work on your own project for a month at a time say.
