I think about a few hours? I usually just leave mine overnight or so and sometimes there's a shaking tree but not all berries give Spewpas (you can get other pokémon like Burmy and Illumise etc) - I believe Persim, Pecha and Mago berries give Spewpas (there may be some others as well I'm not sure) :) If you're finding it frustrating you could always catch Scatterbugs in Santalune Forest and raise them up instead :)