A significant portion of the American population doesn't even vote, so if they can't be bothered to participate in the most basic of political acts, it's a tall order to think they'll watch the State of the Union. That said, the State of the Union is pointless propaganda for the current president. Years ago, Gerald Ford said "the state of the union is not good." That's been true ever since.
Year over year the number of people watching the State of the Union Address has gone down. This is typical of the fatigue that comes along with presidents as their terms progress. More so when a president has won re-election. With only one year remaining, the only thing the President can say is what he has done, not what he can do. This will be nothing but a celebration of accomplishments.
In our current age, more and more people are watching news and entertainment on their own schedules. While I think many will be interested in tonights presidential address, I also think many will be looking over twitter and other social media for the highlights, waiting for relevant clips to appear on their newsfeeds rather than commit to watching the entire speech. I myself will be one of them, waiting for someone to pull out the most important pieces in a form I can more easily digest.
I do not think that a majority of Americans will be watching the State of the Union Address tonight. Most people will go on with their lives and then watch the recap tomorrow of all relevant points in order to be caught up. Especially since it is President Obama's last year, I think people might be thinking that this State of the Union Address is not as important as his previous ones (They are wrong!). Either way though, I do think that a majority of Americans will have seen all relevant parts of the Address by the end of the week though, even if they do not watch it live.