Five of us FlyerTalkers took the tour on the Friday afternoon prior to the September FlyerTalk Getogether. The actual tour is one hour. We probably spent another half hour folowing the tour, hanging out in the gift shop.
Traffic on I-5 can most certainly be brutal and unpredictable. I think our trip up to Everett from downtown Seattle took about 25 minutes

but our return trip took about 2 hours

and that is with me knowing all the secret short cuts.
Allowing 4 hours to drive up there, complete the tour and get back is in most cases realistic. With luck it might only take 3, but it is better to allow an extra hour than to miss one's plane.