And don’t eat at your desk.
I’m not sure yet what length of time is ideal, and it’s probably different for everyone.
But 15 minutes is too little, I’m sure. I’d like to suggest that 30 minutes is the minimum, and that 60 might be ideal.
The most important reason why is to let your mind relax; to get you practiced at leaving problems and stress behind. Unless you’re a first responder, there’s nothing that can’t wait an hour. (And if you’re not and it really can’t, might I suggest that you may want to reconsider doing business with the other party.)
You should also take this time to interact with other people. Splitting these interactions between your co-workers and other people in the neighborhood would be a huge benefit.
If it doesn’t take you the full lunch period to eat, fill the rest of the time reading a book, or blogging, or writing in a journal. It must be something unrelated to work.
By spending your break doing something other than work, you’re allowing yourself to be exposed to things that you think might be unrelated to work, but will actually spark an idea, or introduce you to a new topic or person. This is how serendipity happens.
You need to let a lot of it happen.
And you need to spend a good amount of time in a relaxed state in order for incubation to work. (Maybe that’s why I get some of my best inspiration while soaking in a hot bath, hot tub, or sauna.)