You can certainly add additional flavoring now, just realize that if there are any fermentable sugars in what you add, fruit for example, those sugars will ferment. Also, be sure to sanitize anything you add, unless you're adding flavors from extract.
As far as time, more time allows the flavors to meld/mellow a bit. If it's a high ABV cider, the alcohol hotness will fade out and the cider flavor will come forward over time.
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