Definitely not the 2 day on, 2 off... The half life is like 36 hours(? - not exactly sure), so it's not even out of your system when you are back on!
People usually take it 2 weeks on, 2 off, the theory being that otherwise your receptors will down-regulate (basically the clen will cease to be effective). Whether you agree with that or not should determine whether you go 2 weeks on, 2 off.