When you want to get your song played on the radio, timing matters. No, not just the timing of when your radio promo ends up at the station (though that matters too), but the actual length of your song can have a major impact on its chances of getting played. How long should your radio edit be to maximize your chances of getting played?
Ah, were it that simple. The actual answer to that question is: it depends. Most people asking that question are wondering about getting their songs played on pop, mainstream radio. That radio format is the most restrictive, and the most difficult to break into. If you want your song to have a shot there, don't go over the four minute mark. In fact, try to keep it in the low end of the 3 minute range, or even a little shorter. Anything else is going to take up too much space in the playlist (and eat up too much advertising airtime), so it's not going to make the cut. Now, sure, if when you're a super, mega star whose song simply demands airplay, then sure, push the envelope if you like. They played Thriller on the radio, after all. However, if you pay attention to pop stars that get a lot of radio play, you'll find they stick pretty closely to that timing rule, because, well, it matters.
A quick tip: don't assume that your pop masterpiece just can't be cut and that radio stations are going to swoon over it so much that they'll play it no matter how long it is. Sure, you might win the lottery some day, too, but counting on that is not a very good financial plan, right? Likewise, don't gamble your music career by banking on being that ONE they're going to throw all the rules out for. Things are done a certain way for a reason, so best to just make your song four minutes or less.
Unless, of course, you're not courting pop radio. Other radio formats have more flexibility in their playlists for song lengths. You'll notice that your local classic rock station is more than willing to play Stairway to Heaven in its entirely. Although you can't really make new classic rock, so-called "album rock" stations, that play the classics plus new rock bands are often fine with playing longer songs. The same goes for stations that play genres of music that tend to have longer songs - some types of jazz, some types of reggae, and so on. If you're sticking within the "norms," so to speak, of your genre instead of overstaying your welcome on songs that should have ended two minutes ago, then stations that cater that genre will be open to them.
Non-commercial radio - college radio and community radio stations (including NPR stations) - have the most flexibility when it comes to song length. These stations are much more free to operate under, "hey, I like this, and I am going to play it on the radio" rules than commercial radio stations. Additionally, non-commercial radio stations are usually the outlets for the genres that don't play by those pop rules. Your jam bands, your blues bands, your jazz acts, your bluegrass groups, your math-rock-post-punk-dubstep-screamo acts - these are the folks that find much more willing homes in non-commercial radio than commercial radio, so the rules for cutting length for radio are largely irrelevant.
Ultimately, when you are making a radio edit, you have to consider your market. Stick to the rules for a pop track you're pitching to mainstream radio. If you're playing outside the box, don't send them a 20 minute opus, but don't sweat the four minute mark. In the latter scenario, it's more about knowing when a song SHOULD end than knowing when a radio station needs it to end.