I prefer 42/18.
42 because it is the Ultimate Answer of Life, the Universe, and Everything:
and 18, because it is the Gematria for the word "life":
If I'm going to adopt a system, my inner nerd insists upon increasing efficiency by using components with multiple purposes.
I wrote an Emacs work / rest timer (http://github.com/silentbicycle/zwiebel) for that sort of thing, if anybody's interested. (I hooked it up to an XOSD (http://ignavus.net/software.html)-based alert.)
Pomodoro Timer applet for Gnome (http://linux.softpedia.com/get/Utilities/Pomodoro-Timer-5582...)
RsiBreak for KDE (http://www.rsibreak.org/)
Pomodoro Tasks for Android (currently using)
In fact, I liked it so much, I decided to build my next webapp around it. We're still working on it, but a beta should be out in a few weeks. Sign up for it here: http://www.pomodoroplanner.com
ie, if you work for 25 minutes ala pomodoro, then in the morning or evening do a 25 minute sitting. If you do 42 minutes of work, do a 42 minute per sitting.
It'd be one more thing every day to help train yourself into focusing during the work periods.
It's makes getting back to work much easier.
if you use chrome, it has desktop notifications .. and I added a start/stop to timer..