It takes about a day to fil Hisana's chakra back up, and she'd need to spend three days doing nothing but filling her seal up after using it for training. It's not practical at the moment.
Hm; I wonder about that. I mean GM ruling is GM ruling obviously but consider the following.
As per this canon omake:
"She does at least ten hours of training daily. If it's not kenjutsu, it's her seals, if it's not that, it's healing, if it's not that, it's ninjutsu from her summons. If not that, it's genjutsu on Sasuke. Gai would like her - if she's a genius, she's one of 'hard work'."
Hisana normally trains 10 hours per day. So losing 3 days of training per Mass Shadow Clone session means losing 30 hours of training. Thus the break even point for her would be 30 hours. Arguably it would actually be less since it isn't like she
loses those 30 hours; they are simply no longer available for training and instead must be spent on things like socializing, hobbies, or (clan head)
work but from a purely training perspective 30 clone hours is the break even point.
Naruto produces 999 Shadow Clones for the Naruto Uzumaki Two Thousand Combo. Later in Shippudan during the clone training arc he is using ~200 Shadow Clones to train the Wind Release: Rasengan and complains that due to the Chakra Control requirements that is all he can handle which implies that back during the Wind Nature Transformation training he was using even more. That training session also confirms that Naruto is pulling all-nighters during the training given Kakashi claims he had the equivalent of 4,800 hours of solo training which with 200 clones would require 24 hours of training.
Even if Hisana is only managing a single hour with 30 clones she'd have reached the break even point. If she got that up to 100 clones (or ~3 hours) she'd have
trebled her training time while still gaining +30 hours for non-training activities.
Doubling checking the quest I think the real problem is that Hisana is
already using Shadow Clones to boost her training:
"Oh don't you worry about him. I have something for him that'll blow both your minds." Hisana said, and grinned. Learning the Shadow Clone jutsu had been hard - they really did take prodigious amounts of chakra - but worth it. Being able to have a clone do seal or book work whilst you trained physically was extremely useful, though it was too dangerous to also do ninjutsu training with it out.
That quote for reference is from
before Hisana even graduated the Academy. Back then she didn't have the Final Hurricane Seal (in fact she used a clone to help develop it) which is probably why ninjutsu training was out; she didn't have the Chakra reserves.
Regular Shadow Clones only cost 4 Chakra with Hisana's Chakra Control so theoretically she could summon up dozens of them
already. The real limiting factor is that Chakra is evenly divided between Hisana and her clones.
Lets imagine Hisana can get away with just 50 Chakra for training; which isn't much considering we're talking about a 10 hour session here; that is just 5 Chakra per
hour. Even her
Sharingan uses 3 Chakra per hour and something like our B-Rank Chakra Flows use
6 Chakra per hour. Hisana has a total capacity (natural + FHS) of 365 Chakra. Taking into account the 4 Chakra per clone she drops below that minimum threshold of 50 Chakra by the 6th clone ([365 - 4*6]/7 = 48.7).
Even if Mass Shadow Clone Jutsu reduces the cost per clone to
0 Chakra (which it obviously won't) that doesn't meaningfully grow the limit. Hisana goes from 5 clones with 57.5 Chakra each to 6 clones (365/7) with 52.1 Chakra.
So running the numbers unless Mass Shadow Clone Jutsu allows for the
creation of Chakra (IE: each clone is spawned with equal Chakra to the caster rather then dividing it) it isn't, and likely will never be, viable for training. Not due to Chakra recovery times but simply because Hisana can only split her Chakra so finely and still meaningfully benefit.
Side note but if it takes a day to recover Hisana's Chakra then with 115 Chakra she recovers (assuming 24hrs) 4.79 Chakra per hour. Which means we are officially past the point where Hisana can keep it active 24/7 Itachi style. The base cost is 0.5 Chakra per minute or 30 Chakra per hour which our Chakra Control (90% reduction) drops to just 3 Chakra per hour. It isn't really worthwhile since it means tying up 62.6% of Hisana's Chakra regeneration fairly pointlessly but it is a neat statistic nonetheless.
