Dunno if it helps but the narrator states that 3000 years is a long time in the same exact panel that explains the creations happened quickly.
« And, as if guided by the power of that love, gaining the strength of the friends who had gathered around them, they finally defeated the Demon King, ended the Holy War, and put an end to the
long, long curse. »
The curse lasted 3000 years, a long long time.
This is
fully grown sacred tree (It covers the entire sky across the fairy realm) this is sacred tree once it’s dimension was
created
The calculation mentions the fact that the tree was destroyed
20 years ago and grew back to the
same size it was after growing for 20 years
So either we follow the novel narration that states in the exact same page:
-Creation happens quickly (Soon after their birth they completed it)
-3000 years stated to be « a long, long time »
Else we follow the sacred tree growth timeframe:
-Direct visual confirmation about Sacred tree not being fully grown when he finished his dimension + race
-Direct visual confirmation about Sacred tree growing the exact same size 20 years after it’s complete destruction.
Either 3000 years else 20 years.