Over my career there, yes, I could put a reasonable number of hours on my work and still do 20% projects. There were short periods of time when I was asked to "bank" the time and focus entirely on my main project, eg. I might work straight for 5 weeks on my main project and then take a week straight of 20% time once it launched.

This suited me fine - usually the way I took 20% time was to fit it into time periods when I didn't really have much else to do or I was bored with my main project. And splitting it up like this let me focus more intently on both of them, which helped in delivering.

Cool, sounds like it worked out for you about the way I would expect. I suppose it may depend some on your team, I have heard some people say it is just 20% extra work.