How does one effectively implement the 80/20 rule for developer time?

Question

I am a developer at a bank where an 80/20 rule is implemented. 80% of the time you work on your assigned project and 20% you work on something that interests you. However, I'm not really sure how to do that. I'm hoping there are others out there who can provide me with advice on how to.

My problem is that when I really get 'in the zone', all I can think about is the task at hand. If this is a 20%'er, the last thing I want to do is stop and go, lose momentum and go back to work on my main project.

Am I suppose to work 4 days on something and 1 day on something else? or 4 weeks and 1 week?

I know this depends on the company, but I'm hoping to get a feel for how this is done industry wide.