once, when I was young and stupid, I traded in my copies of Legend of Mana and SaGa Frontier along with my PS1 and eight other games to get $100 off a new PS2
that was the last time I traded in a game
it's not like you'd have to guess
in terms of your music engine, each step of your music will have a well defined length, and it'd be easy to keep tabs on your current length