It depends on what speed you have your game set to.

At 2x speed, a mom nurses for 120 minutes.
At normal speed, 240 minutes.
At 1/2 speed, 400 minutes.

yes, the math doesn't work

You can check to see how much time remains by going to the detail screen for the mom. BTW, if you speed up the time for the mom to get back to work, you'll also use more food from the bin. You might be better off just letting things be at a slower rate, and make sure all of the males are set to the job you want.