Mine is still a puppy so it might not have been fair for me to say 30-40 minutes lol. She has a lot of guard hair on her back, but still has the puppy fluff rough, and pants. The easiest to go through is the hair directly along her spine, and on her tummy. In those places, it seems to be mostly guard hair, but when I brush it is clear that there is undercoat. On her sides, pants and rough, it is thick, dense puppy coat without, or very little guard hair. It goes quickest the more I brush her during the week. I take my time around the sensitive areas like her neck and ears.
We started her out as a little, little puppy letting her lay down on the couch, being fed treat after treat (we made her work at the treat to keep her mind occupied) while I brushed. It worked fairly well, though the only tricky parts are her pants and tail and I still need DH to hold her head for me because she doesn't want those areas touched. It will be better once I get my grooming arm and the tethers (incorrect term?) That go with it.
I am sure that it will take more time once all of her adult coat comes in.