The fruiting speed of stellar plants, although much slower than those of the Floating Land plants, isn't to the extent of taking over twenty thousand years.
As for the maturation time, Song Chi is not yet clear, but what can be surely deduced is that it's highly unlikely to be less than the time required for fruiting.
The urgency wasn't there, as the "Secret Realm Accelerator" in the cultivation secret realm doesn't require any cost. Once this acceleration is over, and after waiting a year, he could conduct the second one. This also means that no matter how long the fruiting speed and maturity cycle of the Silver Apple Mother Tree are, he could reliably acquire the special resource [Silver Apples] in the future.
Of course, at present, only the lowest Fourth Rank Silver Apples, which can promote blue components to purple ones, were obtainable - After all, the Mother Tree had fallen to the Fourth Rank Level. Silver Apples of the Fifth or Sixth Rank were out of the question for now.