Breaking in a mitt properly will take anywhere from a few days to several weeks.
Baseball mitts are very special things.
Far from being ready to use straight from the store, they demand individual effort to fully realize their enormously wonderful potential. Much more than a summer's worth of dreams are contained within that flexible leather palm. There's magic and loss, and the green fields of our first hopes and friendships. It's the nature of leather to adapt to usage, and the only true way to optimize a mitt is by using it to play. What you want to do is soften the leather in the palm of the glove.