So this question has been bugging me for a while and upon a little more research I might be able to give you a relative time frame for when the plane crosses the IDL. But the main sticking point is that the IDL shifts west up to 10 degrees in order to include the entire Aleutian Islands in the north, so again it's dependent on how far north your flight path is. But using a little geometry and making the numbers dance, I would say the plane would cross the IDL about 7 hours into the flight (or 08:25 in SFO). Depending on the flight path and weather conditions, it could be between 6 to 8 hours after departure to give you a timeframe.

Hope that helps.