As a dog trainer I am often asked "how long, how often." And the answer is a simple one; all the time. I recommend for the firt year of your dogs life with you that you are in "training mode," all the time. Now this does not mean that you have to be teaching your dog behaviors the entire time; more that you are in a constant state of educating.
When you let your guard down and allow unwanted behaviors you do an injustice to your dog. Rules are made for a reason and believe me when your dog is 5 years old and up rules will be broken. My dogs are now 8, 11.5 and 12.5 years old and I often give a refresher course.
You are the leader of the pack and as such you make and enforce the rules. Rules can be as simple as "please don't knock me down the stairs," no running out open doors uninvited or sit for a meal." If you have more than one dog then with the addition of a new one; there are alot of rules to learn.
Daily routines are learned in the beginning; where you nap, where everyone sleeps at night, who eats out of what bowl and who gets to eat first, second and last etc. Dogs thrive on guidance, they need a leader and if you aren't up for the role then you will be taking a follower role instead. A dog without a leader is a leader and you really don't want to be following your dogs lead.
Actual behaviors like sit, down, leave it, stay etc are used to implement rules. If you drop a steak on the floor you should have control over that steak; the leave it exercise. I do know many people who cannot have food out on a coffee table for fear of it being scarfed down by the family dog/dogs. Not good.
The more behaviors you teach your dog the more opportunities you have for communication. Stay is a biggy; you should be able to ask your dog to sit or down and stay anywhere. I just can't imagine not being able to communicate my wishes to my dogs. This failure to educate also leads to the necessity of physical control which is in actuality a lack of control. If you don't have any type of verbal or vocal control then you must rely on grab control; correct?
So when you are in the midst of working with a newbie in your pack; be on your toes. Asking for behaviors constantly and working behaviors into your daily routine work much better than taking 20 min. twice a day for obedience and allowing your dog to run amuck the rest of the day. Manners is a constant; dogs left to do as they please only creates a monster you will have to conquer in the future.