How long can female keep lactating to help foster babies

I have a female that became overwhelmed with 12 babies (born 3/4) and some of them became significantly smaller. I split them up earlier this week to a foster mom whom I weaned her own babies early at just over 3 weeks (they were already eating on their own). She took to them nicely and now the smaller ones are catching up to their bigger sibs.

So, I want to know how long I can keep using the foster mom without overworking her. Her own babies were born 2/26 so she's been lactating ever since. I also have a third female that had 15 babies about a week ago and I may need the foster mom again if these babies get neglected by mom. Can I keep using her or does she need a break?

They will do so normally for up to 6 weeks or longer.

Ive had females lactate entire life and would nurse other litters that needed help

Thanks Rich!

I'm agreeing with Rich on this one, I have also had females that just never stopped, just between having their own litters and fostering. But it is probably good to give them a break sometimes, whether they stop lactating or not, I'm sure it is hard on their bodies.