At what point was Celestia's connection to the Elements of Harmony broken?

[ggg-2 3,555]

We know that somewhere over the past thousand years, post Nightmare Moon's banishment, Celestia's connection to the Elements was weakened and, eventually, severed completely. But when do you think this happened? Did, after some centuries, the Elements know things had become so peaceful that they cut themselves off from Celestia, until the day that new wielders needed them again?

[cmarston1 5,964]

Sometime after sending Nightmare Moon to the moon but way before Friendship Is Magic Part 1.

I like to believe that Celestia and Luna both were connected to the elements and with the later gone for a thousand years their connection gradually broke as the many many years went by.  Heck it could have been fairly soon after the whole Nightmare Moon banishment ordeal that Celestia lost her connection to the elements.

[Dynamo Pad 11,689]

I would say right after she used them to banish Nightmare Moon away. Since Celestia and Luna could both use the elements together or separately and since Nightmare Moon was banished the elements were turned to stone seen in the season 1 opener. The elements were probably turned to stone right after Celestia used them or over time since both princesses were in different places or dimensions. The connection probably wasn't as strong when both sisters were together, thus the elements becoming stone and cutting the connection from them and in a way being passed on to the mane 6.

[Steel Accord 6,660]

I think it was directly after she sent Nightmare Moon away. Because the Elements required the wielder or wielders themselves to be in harmony. Without Luna or any true companionship, Celestia would not be able to wield the elements of harmony.

[The Awesome One 1,315]

I think that it did not immediately break when she banished her sister to the moon. She could still use them but not with the same level of effectiveness as before. After a thousand years, she was no longer worthy of wielding the Elements of Harmony and she just thinks one day

 

I need to find new bearers

 

So that is when she is out looking for a pony with potential and she finds Twilight. Takes her under her wing but then she finds out from some old books that Twilight will not be able to wield them alone as each Element would need a separate bearer for some reason (for the plot lulz) and then she sends her out to Ponyville to make new friends.