Composer Max Richter has recorded an 8-hour composition called Sleep, which is intended to be, as the composer says, “actually and genuinely intended to send the listener to sleep.” It’s a long lullaby, designed to sleep through.
At least that’s the hook to get people talking about it. It’s actually an attractive ambient composition for piano, strings, electronics, and vocals.
But if you want the full version, you can get that too, but only by download from the iTunes Store. It’s $35, which is a fair price for an 8+ hour piece of music.
At that length, it might be a bit redundant, but I’m a sucker for long works of music, such as the many long works by Morton Feldman, or piano works like the 4-hour November, by Dennis Johnson. So the idea of a long work intrigues me. Note that it’s not one single uninterrupted piece; it’s a number of short pieces that run together. So you may not like it all, but there may be a lot that you do enjoy.
In any case, if you want a nice, long recording of ambient music, check out Sleep.