I took all sorts of pre-workouts and fat-burners with no real break from them for probably a couple of years, in all honesty. Since then, I've taken a 4-week break back in April of 10, while taking Reset AD and then a 5-week break without the Reset AD this Fall. I noticed no benefit from either one of the breaks. I'm now 5 weeks in on a minimum of 12 full weeks off. I'm taking Revamp (the Reset clone) for the entire duration of this. At the end of 3 months without stims, I'll try a pre-workout and if it doesn't kick in like crazy, I'll keep the break going another month at a time until I notice the change I want.
Once I get to that point, I will never go more than 8 weeks on and 4 weeks off (or more). And that's at normal doses; not something as heavily dosed as an E/C stack. If I were to do that, I wouldn't use a stim-based pre-workout and I'd probably go 4 on/4 off or something like that.