It is well known that consuming spicy foods increases your metabolism and contributes to fat burning, mainly due to the presence of capsaicin.
Since this effect is very small, how many spicy foods would one have to eat (in terms of capsaicin) in order for the weight loss to be noticeable, strictly theoretically speaking?
The increase in metabolic rate from ingesting capsaicin is nominal and would make very little difference to someone trying to lose weight.
Of more interest is the fact that capsaicin appears to increase satiety - it increases the feeling of "fullness". Studies have shown that adding capsaicin to food results in the overall consumption of fewer calories. This effect too is relatively small, and so would likely only have any practical effect if the eater were also carefully managing their calorie intake.
Eating more spicy food would not cause you to lose more weight - you would simply be consuming more calories. Attempting to add more capsaicin to food in order to increase the effect would simply result in unpalatably hot food.
So, all in all, whilst there does appear to be an effect, it's small. I don't think any amount of spicy foods will cause weight loss on their own - only a planned diet will do that. In other words, spicy food won't cause you to lose weight, but if you are trying to lose weight, spicy foods may well help you to feel better while doing so.
