take as much time off as you spend on. Too long on anything isn't good, besides your body adapts to what your taking making the supplement ineffective. In terms of side effects, irregular heart beats, dizzy spells/fainting are some you should look out for.
Slightly off topic: To have only dropped 10lbs in 12 weeks as a drug free trainer means you have plateaued and upping the dose or changing brands I wouldn't recommend. I'd recommend changing your diet and reviewing your cardio program.