If your dog is expressing the anal glands on his own naturally, don't mess with them. Once you start manually expressing the glands (on a regular basis) you will have to continue manually expressing the glands. It's one those "if it ain't broke, don't try to fix it" kind of things. A dog *should* express their glands when deficating. The amount expressed will vary, and they may not do it every time they deficate. A dog will also express their glands when under stress. It happens more often than not when we take a temp on a patient at work, and cats are notorious for spraying techs in the face (oh what fun). Normal anal gland secretions are thin liquid in consistency, and a light to medium brown in color. Any "chunks" or a consistency that is "thick" or "pasty" (think toothpaste) is abnormal and needs to see a vet. It always smells bad, it's supposed to. The anal glands are scent glands, and generally act as a passive territory marker. As for how much the glands can hold, it depends on the size of the dog, but generally speaking... a lot more than you would think.
Small breeds and those that are overweight tend to need their glands manually expressed more commonly than large breeds and and those at a proper weight.
Vi had to have hers expressed every time she came into heat. Since she has been spayed, I have only had to do them once when she had pancreatitis. Sune (my Chi) has to have them done every 6 weeks, and we are still battling impaction issues from 3 years ago when I got her. Lucy has never needed hers expressed.