Soon as the park opens! That way you'll be absolutely guaranteed to get in.
Sorry, I couldn't resist.
Realistically there are WAY too many variables to give you an exact time to show up. It depends on who is doing the reading. People line up five hours ahead for Neil Patrick Harris. And occasionally they walk up 15 minutes before the show starts, on a quiet weekday night with an unknown reader. If you're one or two people, you're more likely to find a last minute seat, than if you're a party of 12.
Assuming you're not trying to see NPH, I'd stroll by and take a peek at how the line looks around noon. If it's small, I'd check it again in an hour, and again, an hour after that. Then every half hour. Jump in whenever it starts to look long.
Alternatively, plan on seeing the final show of the night, get into the standby line 15 minutes before the show starts, and hope for the best. If there's no seats left, at least you haven't wasted much time.
Or just stand behind the theatre and watch it from there.
OR, just do what we do, and get yourself a dining package. It's a wonderful excuse for a sit down meal and you don't have to get in line more than half an hour before the show (make it an hour, if it's NPH).