If you have any doubts about how effective short training sessions are, check this out:

According to workingaussiesource.com [1], Jay Sisler trained for 10-15 minutes, three times a day. He worked on just one thing at a time, trying not to confuse the dog. If the dog wasn't getting something, he would stop working on that trick for "a month or two" then come back to it.

How good was that part where Sisler and his three dogs jumped rope together?