I would assume (with a good amount of certainty) that charging just adds frames to the smash's startup, and the hitbox data progresses as usual upon the first frame of button release.
So using DSmash as an example again;
The smash charge window doesn't open up until F16, and the hitbox comes out F20. That means that holding A from F1-F15 doesn't do anything in terms of charging, and holding it starting on F20-onwards won't affect it either. You have to press A F16-F19 to get a charge.
Say you press A for exactly 1F, on F16 (I'm assuming that even a 1F input in within the window is enough to register a charge; that may or may not be the case. The lack of frame advance for this game makes it hard to say and I don't remember if that happened for Brawl). This means that the release frame is on F17, rather than F16, as it normally would be. Since the hitbox comes out 4F after the release frame with no charge (F16 + 4F = F20), the hitbox would come out on F21 here (F17 + 4F = F21).
Like I said, I'm not sure if there is a threshold for the number of frames the button has to be held for charge to be initiated, but in any case I'm pretty sure it would still go like that. If you charged from F16-F19, the release frame would be F20, and the hitbox would be out F24.
Idk if that answered your question. You asked how much of the startup charge covers. That just depends on how long you actually hold the button to charge the smash. If you meant how many frames you have to begin charging, that would be the first active frame - the charge frame. So for FSmash that's 14 - 10 = 4. F10, F11, F12, and F13 are the frames of the attack when you can begin charging it. Any inputs before or after those frames don't make a difference. Of course, if you press A on like F8 and HOLD IT TO F10, that works (when I say "hold it to" I mean you are holding the button on whatever frame you held it to). But if you release it on F11, you only got 1F of charge, not 3. If you release it on F10, you don't charge it at all.
