The number one question I'm asked as a First Dance Instructor is how long the first dance song should be. Although my quick answer is about 2 1/2 minutes, there's actually no barrier to how long your first dance should be! The choice is totally up to the two of you -
Here's a few factors when considering the length:
Your song choice. Sometimes the lyrics are too meaningful to shorten the length of the song, if that's the case go ahead and do the whole thing!
Your comfort as a couple on the dance floor. Some of my couples want to know when's the soonest they can get off the floor, while others don't mind the at all.
How long the song is naturally. Sometimes the songs are already on the shorter end (under 2 1/2 minutes) - while others can be exceptionally long (over 4 minutes). The sweet spot should be somewhere over 2 minutes but under 4 minutes.
Knowing what to do. Rocking back and forth middle school style is romantic and also simple. If you find yourself bored after a certain point as the song is playing, fill the time up with some spins and other moves. If you're not quite sure what else to do, look up instruction on YouTube or seek a local ballroom dance professional to customize it to your song.
Just remember there is no wrong answer, the first dance is yours and represents you as a couple. Go with what feels right for you and you should be all set!