Probably not. First, the weight to use is completely dependent on your strength level. Secondly, only doing flat bench press is very one dimensional. Although some people live and die by the exercise, and I def. think it is one of the "monster" exercises, I, personally need to do more then just flat. I like to implement some incline and decline exercises into my routine as well to work the chest from all angles.
Flat bench, believe it or not, does target the entire chest. However, you can focus more intensely on the upper and lower chest by working in some incline and decline presses and probably reach your goals a lot sooner.
If you have never exercised before, flat bench press may be good enough for now. If you are looking for muscle hypertrophy though, I would increase the reps. Try for 8-12 reps using a slow concentric and eccentric movement, no pause at the top or bottom.
I hope you are working your other body parts as well, not just chest!
As I always say, your muscles will adapt, so doing the same thing over and over is not the answer. Switch things up every so often.
And none of this matters if your nutrition is not right!
Hope this helps.