When I work in post, and I need to mix in music so it's at "broadcast levels" I mix it in, ie. turn it down, so it's average is around -20. With the way commercial mixes are, there might be as little as 6db in peaks over that, so it might only peak at -14 to -10 usually. However I often deal with music that has not been squished in mastering.
It's all about reading VU/RMS meters as opposed to Peak. Oh , and your ears.