I work with shelter and rescue dogs and had just recently completed muzzle training with two of our aggressive dogs in our rescue.
One of them is more leery than the other, so you would have to go at the dog’s pace as well. I use a video link by Mike Shikashio, (aggressive dogs specialist) and it took us at least 2 months to finally get one of the two dogs wearing the muzzle fully strapped. Of course, we are doing the training about 3-4 times a week, not daily since we run on volunteer help. Not days…. Very little baby steps.