Hmmm, good question. 3 hours would be a safe bet (usually a 2.5 hr trip) without stop 'n' go access road traffic, road closures and a choked I-5 between Everett and Seattle, and Seattle and Sea-Tac. I haven't made that trip with an itinerary though.
However, I have made the Stevens Pass trip many times. It is a usual 1.5 hours. On Sunday afternoons I give myself 2.5 if I HAVE to be on time. Every once in a while it is not enough (road closures, accidents, the Sultan stoplight
: ...) Stevens does get a lot more traffic, but on the other hand Baker gets a lot more snow, the road basically gets down to a single lane and they have time limits on up & down traffic on the upper road sometimes.
My .02 would be to add an hour to the normal time frame just to allow for conditions and traffic etc.