Building the leak from the ground up.
Let's start with planning, he would have either had to come up with a leak, or find a leak that's believable, or not popular enough to get noticed, and take inspiration from this, changing bits here and there to make it seem like it's not an entire rip-off, this, being the planning phase, this is to assume it's one person, could take a few hours.
Next, we move onto renders, which, I'm no artist, but the full character renders must have taken some time, as they had to line up well with the artstyle of this game, fit over a base of the menu, and still resemble the original character. This could've taken days, just for the newcomers.
Now, we move onto veteran renders, of which we have 8 different un-announced veterans. For these he would've either had to completely create these renders from scratch, or use a base, that, for most of these characters, can be identified, and touch it up so that it looks real enough to pass.
Finally, he would have to come up with the GUIs, as we haven't seen the settings on the bottom screen before, or the title screen, or the full SSS or CSS. This would've taken a fair amount of time, keeping in mind that all of this had to be done within the time that the Ninka leak happened. So, either our friend is a ninja, or they are not alone, and we have multiple people all working on one big leak.
That is, unless this is real, which I hope to god is.
Now, these CSPs were well done if fabricated and each could've taken a day, and if he already had a Meta-knight CSP ready before the reveal, he would have had to scrap that entirely, wasting another few hours to a day that it took to make the Meta-knight, and crop the new CSP to fit in the menu. Each of the new characters could've taken a day to make, if made from the ground up. So, as of right now that puts us at 4 full body newcomers, and one icon of one, which could easily just be a modified Pit. This would make up for the time lost on Meta-Knight, so if we are going to find a flaw anywhere my guess is it will be on Dark Pit, who was probably thrown together quickly if this is fake. So, that puts us at 4 days of newcomers\Melee Vets, some lost time made back on Meta-Knight with Dark Pit, and now for the veterans we have ROB, who could've taken a short while to edit, as we only see his face, which wouldn't be hard to touch up, my guess is 1-3 hours (again, I'm not exactly an artist, I don't know this stuff), Wario is under the same scenario, as we already know he is from SSE, so another few hours, G&W was probably easiest to fake (if he was) due to his few frames of animation. Ness seems to be a completely new portrait, which makes sense when you think about it, as he is the only non-melee unnanounced vet to show up with a full body CSP. So... make of this what you will, have fun!
EDIT: Dark Pit easily could have been where Meta-Knight was, but he changed it when MK was announced, so he didn't look sketchy.