Just got back from Napa in October. We had three days for tastings.
We did one winery the first day (plus French Laundry); three the second; and one the third, with a tasting stop at the end of the day at RH Vault in Yountville.
Since we had a group, everyone got to pick one winery so we did Inglenook, Caymus, Frog's Leap, Stag's Leap Cellars and Silverado. All were enjoyable.
That second day was a little tiring but definitely doable.
Make sure you have reservations. Some of these wineries expect you to pay in advance, too.