For an Uchiha to "evolve" their [Sharingan] into the [Rinnegan] was extraordinarily difficult.
First.
That Uchiha had to be Indra's reincarnation, with Indra's chakra inside them.
Second.
That Uchiha also had to obtain Asura's chakra.
Finally.
The Uchiha needed to merge Indra's and Asura's chakra to resonate with the Sage of Six Paths' chakra and thereby awaken the [Rinnegan].
As for how long merging Indra's and Asura's chakra would take...
There was no way to estimate it.
At any rate, as far as Uchiha Minamoto knew.
From the end of the battle at the Valley of the End until just before his death, Madara Uchiha had only then awakened the [Rinnegan].
That had taken at least several decades.
And that did not even rule out luck as a factor.
Because in the Shinobi World's thousand-year history, only that single pair of [Rinnegan] had appeared since the Sage of Six Paths!
And over those thousand years.
How many reincarnations of Indra and Asura had there been?
So.
It was really hard for an ordinary Uchiha to "evolve" their [Sharingan] into the [Rinnegan]~
"Lord Minamoto, aren't you surprised?"
Mei Terumi could not help asking.
"Those are the Sage of Six Paths' eyes!"
"What good would being surprised do?"
Uchiha Minamoto set down his teacup and glanced at Mei Terumi.
"Would my [Sharingan] turn into the [Rinnegan] just because I acted surprised?"
Mei Terumi was left speechless.
Only after a long while.
Did Mei Terumi manage to speak, raising her right hand like a blade and drawing it across her neck.
"Since the leader of the Akatsuki has the [Rinnegan], should we..."
"Strike first? Seize Pain's [Rinnegan]?"
Uchiha Minamoto looked at Mei Terumi and clapped lightly.
"Great idea!
"I'll leave this important mission to you, Mei Terumi. I've got high hopes for you~"