Depends on the fat burner. Clenbuterol (a beta 2 agonist) worked best for me, but its illegal, fairly risky to use and relatively hard to get. Also you could eat above your approx maintenance level and lose 4-5lbs a week of fat, no joke. Another downside is it only works for about 10 days to 2 weeks.
Ephedrine works in weightloss by thermogensis but mainly the anoretic effect, i.e. appetite suppression, at the later stages of calorie restricted diet it could boost the fat loss which slows down as the body adapts.
Typically it might as much as double your normal fat loss per week in some circumstances.
Despite what people seem to think, Ephedrine doesn't lose all its effectiveness until very prolonged use, but the bad side effects do dissapear. Typically it'll work well over the course of a decent calorific restriction period.
Also it can counterract some of the strength losses from calorie restriction as it is aids strength a bit.
As far as HOT-ROX is concerned I think you have to use it for a couple of weeks to start maximising the effects, probably by then offers simialar fat loss potential as ephedrine and caffeine in moderate doses, but probably a bit better and less side effects.