Most wineries typically run their tastings to be about an hour or hour and a half, pre-covid some were a little more relaxed about time frame some still might be, it certainly helps if you intend to buy wine. Unfortunately wine tasting in Napa has kind of become more of a sales event where turnover matters.

I'd recommend Round Pond, Alpha Omega, Del Dotto, or Turnbull (all Bordeaux varietal heavy places) as these have typically been places where I know time limits are more of an afterthought. For sparkling outside of the majors (Mumm, Chandon, Gloria, Domaine Carneros) I'd recommend going to Mendocino or Schramsberg, otherwise there isn't a ton of sparkling in Napa.

Finding a winery that sells buy glass and allows picnicking might be the best option if you're looking to enjoy a few glasses over a couple hours, not all wineries can serve by the glass or allow pinicking as it requires a separate license so. I'd check with the specific ones you're looking to go to.