Hmmm, I think it would take you maybe 1.5 hours, maybe two to the top. There are some nice picnic areas, and a really nice Visitors Center at Chilao Flats. We usually like to go to Buckhorn, which is higher up, but they have no running water - instead, they have a huge tank & you get your water from there.
Are you looking to just drive through there, or stay for the day for a picnic, or camp up there?