I dunno its kinda debatable...
As long as your are keeping infusions short for the japanese green's , and you are adding after each infusion (not just after the tea starts to fade at #3 or 4), then you could really go on pretty long. Rule of thumb for me is not to let combined infusion time go past roughly 6 mins-8 mins. I use this method for drinking senchas, kukichas, banchas, kabuse etc when there's a lot of people who are thirsty. If done well the infusions taste good (kinda a cross between #2 and #3 when brewed 'regular', not flat, muddled, and lifeless like you may think. I can comfortably get about 5-6 infusions like this, more if I pushed it, but I don't like tea wateriness too much anymore.
Why not get a little bit more of that umami each infusion instead of just enjoying the majority of it in 1 and 2? I would just be careful not to add too much leaf each infusion, and to also be very diligent about not letting water sit in pot in between infusions (as more leaf=more mushy=more unpleasant flatness due to excess water and leafs sticking to each other, so I swirl very gently during brewing).
Much of this stems from my opinion changing sessions of gyo kukicha, blended with first flush bancha. It was great being able to adjust the balance of what I liked about that blend each infusion. Want more earthy next cup? sure. how about some more quite calm for the next one? Why not? Really opened my eyes to the possibilities of trying non standard things. Sencha is a little more of a tricky animal then a blend of opposites, since the balance is less easily manipulated and leaves are more delicate, but still doable.
Oni wrote: I would prefer putting enough leaves at the beginning, around 5 grams / 100 ml
So do I, but sometimes I change things up to accommodate for time, available teaware, available tea, #of people I am brewing for, curiosity, and sometimes, but rarely, pure laziness not wanting to wash two sets but 500ml is not enough to quench the thirst.