I'd say the same as others 3-4 weeks is the normal weaning age and once the pups are taking solid food they do not need their mothers milk. Some mothers or pups are reluctant to finish taking milk at such an age and will continue or actively pester mum for milk.
Personally I feed pups up to 4 mths of age 4 meals a day and 2 of them consist of milk ( one just milk and one milky cereal) you can use puppy milk, goats milk or cows (if refridgerated let it warm to room temp before giving it to them), but again that is also debatable by some people. It works for me and the pups are happy with it. If the pup brings up a bit of phelgm after milk you can start to add a teaspoon of honey to it as this will stop that problem.