Creating a music playlist that is only 30 min

im a complete beginner, so bare with me!

Im a guitarist who wants to create himself a playlist of songs to play from all songs that I know...Knowing a couple now I just want to play for 30 min.

For that my idea is to create a dictionary with song title and duration of the song. I wanna make a random choice of songs, whoose duration doesn't surpass 30 min. My issues are how do I write down the minutes and seconds of a song.ç

My idea is:

Playlist = {"Before I Forget": 4.21, "Unsainted":4.20.....}

But that stores the information as a float...Any advice?