Some dogs (like ones that are over motherly) & certain breeds (like aussies) can be known to let ther pups nurse for as long as they want.
If the mother does not start getting restless with the pups nursing at 5 weeks (maybe a week or 2 longer for very small breeds or if their teeth are not developed at all yet) of age you should step in.
Start seperating the mother & puppies more often and offer moistened kibble frequently to the pups. When they are 6-7 weeks old and the mother is still nursing & often you should remove the mother completely. Of course keep a very close eye on them & make sure they are eating the kibble.