We've had grocery stops with Tiffany Towncar three times and with Quicksilver Tours once. Each of the drivers stopped at a different Publix store, so there's no guarantee where you'll go.
We always had grocery lists ready. Because we were stocking our kitchens at DVC resorts with food for a week of breakfasts, a couple of dinners, snacks and beverages, we always needed almost the full 30 minutes. Keep in mind, that it takes longer to find things in an unfamiliar store, and you have to allow time to check out.
When Tiffany cut their official grocery stop time to 20 minutes, we switched to Quicksilver.