It normally takes about 5 1/2 hours to get to Seagull Lake from the Cities. Because of its curves and the possibility of moose licking road salt, I would think the only added time might be on the Gunflint Trail. That is unless the roads are bad because the weather. I would estimate a 6 hour drive time with good road conditions.
We left at midnight from about 45 miles south of Duluth a couple years ago and we got there about 4 AM. We took it pretty easy though because it was right after a decent snowfall and the gunflint had some slick spots. It really depends on condition, I kept it in 4 wheel the whole time on the gunflint and I still slipped around a couple of those corners. I try not too rush it though as I would prefer to not have my trip ruined before it starts.