Complexity is going to determine how fast you should/can play a song. Watch Tommy Emmanuel and you'll see someone who can play very complex material at blazing speeds. Most of us don't even pretend we can be another TE. But then, most of us aren't playing TE's material.
If you can play most songs at 120 bpm cleanly - really cleanly, you can play most songs.
If you can play most songs at 180 bpm, you can play most anything other than what Ike Turner recorded.
If you can play a song cleanly and with interest at 40 bpm, you'll have accomplished a surprising feat IMO. It's more difficult to play a song slowly and cleanly than at, say, 120 bpm where you can let slide a lot of your fingering mistakes.