We let Chloe nurse her pups (who were staying with us) as long as she wanted, which turned out to be until about 14 weeks. She never indicated any displeasure about it though. I would make sure your girl can get away from the pups if she wants so that she has the option of leaving instead of nursing - maybe use a low enough ex-pen or gate that she can jump out but the pups can’t. As for the rash, you should probably get a vet to look at it to make sure it’s nothing more than just dermatitis (might be yeast/thrush for example), but to protect her skin I can’t imagine why you couldn’t use the lanolin that’s available for nursing human mothers (Lansinoh is the most common brand name).
I’m assuming you’re not keeping all the pups so they will wean once they go to their new homes. There are some breeders who would insist the pups be forceably weaned by now (mostly by separating them - you can also put a onesie type shirt over the mom so they can’t access her teats), but personally, I didn’t see the need.
Thanks for fostering!!