The stress magic puts on one's mind is relative not to its power, but to the magician's skill or aptitude for said magic and its complexity. Miyuki's talent is so great that she can cast magic like Niflheim and Inferno, which even A-rank magicians have difficulty casting, with ease. Possessing innate magic means that you possess aptitude for said magic, which means that you'll have an easier time casting it than someone who's not cut out for it. Tatsuya was born only capable of Regrowth and Decomposition. It's only natural to conclude they're easy to cast for him. In fact, it has never been stated that he had any difficulty casting them.
You pretty much just agreed with what I said about innate magic and not actually answered what part of the novel made you think Tatsuya could spam his magic as much as he wants. Magcians are stated to get mental exhaustion and psion exhaustion when using magic in the novel, and we have already seen the same a few times for Tatsuya.
You say that Regrowth is a powerful magic, but its a relatively simple operation and very similar to Data Fortification that everyone can use.
Miyuki gives a grand description in v8 of how powerful and massive the spell construction was. The novel never described it as simple, but a spell that required a vast information aide. As a magic spell, it cannot be compared to data fortification since it actually rewrites the eidos instead of just reinforcing it. Its so powerful the novel states it effectively rewinds the time of the target.
Also, Tatsuya's used Regrowth and Decomposition for battles that lasted hours and we've haven't seen any side effects.
It took near a decade before Miya had to be hospitalized. A few hours means nothing really except that if he overused his magic, his overally life span would be shorter.
And I don't remember anywhere where Tatsuya used his magic constantly for hours. His magics are so overwhelmingly terrifying his battles end in minutes. In v7, compared to Ichijou and Mari's and Miyuki's group, Tatsuya barely spent time fighting due to being part of a strong special forces unit and his ability to easily steamroll his opponents.
I remember in v7 when the 101 finally went for a full assault, the enemy ran after 15 minutes, and Tatsuya didn't even stay to the end. The v8 battle most likely ended just as quickly once he started in.
In Genzou's case, he ran himself ragged for months taking down Dahan until he pretty much burned out his mind. It wasn't from overusing magic that was beyond his ability to cast, but overusing his own innate magic straight for a prolonged period of time beyond his limit.
This agrees with what I said. The novel states he overused his innate magic. In this novel it states the mind has a limit on magic use and the novel also shows once the mind starts to fail, the body follows. His and Honami's case are just more extreme as they were in live or constant live combat.
In Miya's case, she had overused it over the course of several years until she died of weak body. Not a burned out mind like Genzou.
The novel also states she overused her innate magic, so if we go by the novel descriptions its the same case of reaching the limit of the mind's magic use. Once the mind fails the novel shows the body becomes exhausted and also starts to fail. It just took longer in her case.
Honami's case more similar to Miya's than Genzou's. Due to her artificial origins, she already had a small life span and an unstable life force. The one time overuse of magic was enough to burn her out.
They are all stated to be the same as the novel describes the main problem in every case as magic overuse. The definition of this given in v8 of the novel is 'exceeding the limits of the mind's ability to use magic'.
This is how I understood it. Honami died much more quickly because of her artificial origins and subsequent much shorter natural life span. Genzou took longer because his origins were of a more natural one compared to Honami, and so originally he had a much greater natural life span. Miya took the longest due to the less stressful nature of not having overused magic in constant combat like the other 2, as well as also having a much greater natural life span than Honami due to Miya's more natural origins.
In all 3 cases their bodies would have likely just stopped to work like in Honami's. Unfortunately, I can't say for sure since we don't know of Genzou's and Miya's actual last moments. Although in Genzou's case I do know his body would have burned away due to conditionally activated magic.
Indeed it was never stated that regrowth is a powerful magic and it causes huge burden on his body
Yes the novel does state more than once it is powerful. And it doesn't matter if Regrowth specifically causes a greater burden really, my main point was that if you use magic constantly, the novel shows that there comes a point where you may shorten your life span to a dangerous level. From the 3 magic overuse examples shown in the novel, the magics involved were their innate ones.
simply put he never had any side-effects .
Because there has never been stated to be any early side effects, It took years of overuse for Miya and half a year of overuse for Genzou. Once you go over, its too late and I assume your body also starts to immediately give out. Thankfully Tatsuya has NOT been trying to make himself immortal.
Moreover at the time when honami was dying what i understood was that she died only when tats let her go. her heart might have stopped but using regrowth he was reversing it but she didnt have any time left so she was in a way trapped in the loop if he didnt let go .
Although I don't really think so since the novel mentions Tatsuya only tried to use his magic but then realised it was futile, what you have mentioned is also very likely. But we do know she suffered no damage except the described mental one. Her body was just described as being exhausted and she just stopped breathing.