I'd say in off season you'd be fine about 20 or 30 minutes before. At least cruise by and see if the show is beginning to look crowded. If not, you can shop a little (there are a few kiosks of merchandise) and maybe see a character or two, if any are out, but really, once you're over there, there's not much to do (unless you want to see the Pocahontas show and them go into Lion king after it's over). In the off season, I don't think it will be so crowded that you'll need to be there 45 minutes or an hour ahead.
It's a wonderful show - my very favorite at WDW!!! Enjoy!!!