I'd say try for a PS at 7:15 or so.
This may sound really early, but it's not uncommon to have to wait a bit at 'Ohana. Also, it's the kind of meal that you don't want to rush - while the skewers are fabulous, my favorite is actually the wontons from the appetizer plate.
Eat a bit, let it settle, eat some more, wait a bit longer, . . . . . . hmm, maybe 7:00 is more like it. Even if you're done early, the Poly is one of the best places in all of WDW to just stroll around and take in the atmosphere.