Does it look like fighting is going on at the entrance, or is it just clogged with lots of bees? Are you using a Boardman entrance feeder, which can incite robbing, or an internal feeder?
I've never had to close up a hive like that, so I can't speak from experience. But, since you have a SBB, ventilation should not be a problem, barring some really warm weather. However, they will need food and, being active now, they will have to relieve themselves. I wouldn't go more than a couple of days with the hive closed up.
Others, with better experience than me, will give you additional advice, I'm sure.
“If you want to gather honey, don't kick over the beehive.” - Dale Carnegie