From VS Website
http://www.virgin-atlantic.com/gb/en/travel-information/airport-guides/sanfrancisco/experience-upper-class.htmlKiosk Check in
Our automated kiosks and bag drop provide a speedy option for check in. You’ll find four kiosks next to our check in desks, which open six hours from departure. When you’re done simply drop your bags at desk 25.
Counter Check in
For our regular check in service head over to Zone 10 of the International Terminal, where you’ll be met with a cheerful smile and slick service from our staff. Check in opens at 15.50 on Monday, Tuesday and Saturday, and at 15.00 on other days.
Provided you don't need a human, it seems you can check in 6 hours before.
From the VX website
https://www.virginamerica.com/cms/fly-with-us/vx-lounges/sfoHours of Operation Tue, Wed, Thurs, Sat, Sun: 2:00 P.M. - 8:30 P.M.
Have a good flight!