A good regime is 15 minutes per day. This is not too hard to manage for the owner, and not too overwhelming for the dog, but it's enough to get good results. You can split it into 2 or 3 shorter sessions.
It's good to keep up a daily routine, but don't panic if you miss a couple of days, or if every day is just too much for you. You can come down to 3 or 4 times a week and still be effective.