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.