Naruto: Reborn as Sakura with Manyuu Chifusa Template

Sweat dripped down Sakura's forehead.

Perhaps she had judged the original Sakura a little bit too harshly— condensing the Yin Seal was much more challenging than she had erstwhile thought. No wonder she never paid any attention during combat, if she was constantly concentrating on making sure that her head didn't implode on itself…

Even though Sakura was no stranger to multitasking (since shinobi often had to mould chakra during combat, in case they ran out and had to make more), consciously separating a stream of chakra and sealing it within a tiny diamond on her forehead was on a whole different level of complexity.

'16%... 17%...' said Inner Sakura encouragingly. 'We've got no time left, and we're less than one fifth of the way there! Keep at it, me!'

Knead chakra, eat, sleep— repeat cycle.

For the past seven days, she had done nothing but knead chakra and store it away. It was mind-bendingly tedious work, but Sakura didn't have the luxury to allow herself to get bored. An unbelievably precise degree of chakra control was required to channel this much chakra into her forehead— if she were to make a single mistake, the seal would come undone; and at best, she would lose all of her progress.

"Ugh, I think might have overestimated myself," Sakura groaned. "I hope it gets easier as I get better at this… How much longer do I have to hold on…?"

---IT'S THE BUSTY KUNOICHI TEACHER SAKURA'S MATHS CORNER! ----

According to the Databooks, the original Shippuden Sakura scored a 2.5 in stamina, while Kakashi scored a 3. Since this man is the basis for our measuring system, we know he is worth exactly 1 KK.

So, going by these numbers, and this is being very generous, we can calculate that Shippuden Sakura possessed a 0.8 KK chakra pool; which is an increase of roughly 0.6 KK, compared to her humble beginnings.

In other words, after training for two entire years under the Fifth Hokage during the time skip, original Sakura had gained the amazing ability to use the Chidori a grand total of three times… had Kakashi bothered to teach it (or any other Jutsu) to her in the first place.

…

At any rate, knowing the amount of chakra at the original Sakura's disposal, Tsunade had calculated that it would take her three years to condense her Yin Seal— and taking into account how debilitating it was for Sakura to do so by watching her training episode, we can assume that she was sealing away most of it— maybe even as much as 95% (there is no way to verify this value for certain).

However, this is not what had ended up happening.

The original Sakura had refused to sit on her laurels and do nothing but accumulate chakra during three years, so she had slightly modified the Yin Seal, making it even more complex. As a result, instead of taking her three years to complete the seal, it had taken her four, but in exchange, she had retained her ability to perform Iryō Ninjutsu, as well as her combat effectiveness (lol).

…

Now that we know the original Sakura's stamina pool, the question we must now ask ourselves is: What is Sakura's stamina regeneration rate?

Luckily, there is a way to approximate this value; yet again, using Kakashi as our reference point.

After exhausting nearly all of his chakra during his first fight against Zabuza, we know it had taken an entire week for a bedridden Kakashi to regain all of his strength. Even though Kakashi is extremely talented, he is only human: he doesn't possess any special bloodline, just like the original Sakura, so we can use him as our baseline.

Each human cell being more or less equal, we can assume that it takes a full week for a regular shinobi's ATP levels to be restored to normal after a near total chakra depletion— so, whether you only have 0.8 KK or 1 KK, we can safely say that it takes 7 days to completely refill the energy stored in the human body's cells— no matter how many KK one happens to have.

(In Boruto, Sasuke constantly runs out of chakra after using chakra-intensive Jutsu and consequently has to operate on low-energy mode for days on end, so we know that this week-long approximation is more or less accurate).

…

Now, we have all the variables at our disposal, we can calculate the Yin Seal's maximum capacity.

Using Tsunade's initial plan for Sakura; which is to say, kneading chakra every single waking moment of her life, and sealing over 95% of her 0.8 KK chakra pool that fully regenerates every 7 days over three years, we can approximate the Yin Seal's maximum capacity: 120 KK.

(Since the original Sakura had ended up taking one additional year to fill up her Yin Seal [instead of going along with Tsunade's 3-year training plan], simple maths tells us that instead of sealing away 95% of her chakra, she would have only sealed away around 70% of her weekly supply; keeping the remaining 30% to go on adventures with Naruto or to help with surgeries.)

Needless to say, Sakura couldn't afford to wait three or four years to condense her Yin Seal— but neither would she have to!

Thanks to her acquisition of Tsunade's breast chakra, Sakura now possessed a mixed Senju-Uzumaki heritage, causing the rest of her cells to become filled with extreme amounts of vitality.

Unlike most shinobi, people like Tsunade and Naruto could exhaust all of their stamina, go to bed, and wake up fully refreshed the following morning, instead of having to wait a full week like ordinary people.

For example, Naruto could use up all of his energy during his various training sessions (Tree-Climbing, Toad Summoning, etc.) and sleep the fatigue away, before resuming his training the very next day.

As for Tsunade, it had taken her a single day to go from shrivelled fig state to her ordinary self at the conclusion of the original 'Search for Tsunade' arc, after expending so much chakra for the Creation Rebirth Jutsu that she had lost the ability to maintain her permanent Transformation Jutsu.

With this last bit of information, we now possess all the variables required to calculate how much time it would take for Sakura to charge up her Yin Seal.

Knowing the Yin Seal's maximum capacity of 120 KK, Sakura's available stamina pool of 3.52 KK, and that it takes her one full day to regenerate all of her lost chakra, we can deduce that it would take the current Sakura 36 days to completely fill her Yin Seal, as long as she dedicated 95% of her chakra to the seal daily.

Indeed, 36 days was a much shorter period than four years, but unfortunately for Sakura, she had run out of time— Hinata's wedding would be taking place tomorrow!