"Can anyone tell me how long I should wait before I fastfall and L-cancel or If I should do it faster."
I do not believe there is a way to transmit information about how long to delay apart from experiencing it yourself. Sometimes there might be occasional alternative methods to try which will yield better results due to some method. For example, using control stick to jump from shield into pressing B has a distinct advantage over say Y -> Up+B since you have to time three inputs with three separate fingers and that can lead to mistakes.
But, for the case of getting your L-cancel despite hitting light shield, nothing, shield, or an opponent I am unaware of any sort of mechanical method you can try to improve your success. I believe this is the case of literally practice makes perfect.
I can tell you that different situations might require you to fast fall and other times to not. For example, if you hit someone on a platform, its not always best to fast fall right away. Its preferable to wait and strike when they release shield or perhaps you can poke them by falling down a bit. But, you cannot get the right timing if you fast fall prematurely. Similarly, SH Double Fair cannot happen if you fast fall mid short hop. Likewise, since hitting an opponent cause hitlag and freezing you in the air your L-cancel timing must be delayed. I believe hitting a light shield gives the most hitlag to you. While shield and hitting a target have the same hitlag for you.