You want a number. The honest answer is a range, because adolescence is a stage rather than a birthday.
The range
Adolescence in dogs runs roughly from 6 months to 24 months. Larger dogs generally take longer than smaller ones, and there is no test that tells you your own dog has finished.
Reduced responsiveness to the person who raised the dog appears around the onset of adolescence and returns later in the stage. That one finding explains most of what owners describe as a dog who has stopped listening.
What tends to happen when
6 to 9 months. The first surprise. Recall gets patchy outdoors, the leash gets worse, and guests start getting jumped on. Owners usually read this as their own failure.
9 to 14 months. Generally the hardest stretch. Distractions win, the evening settle is hard, and barking at the window may start. Your dog is now strong enough to make all of that matter.
14 to 18 months. Things get quieter in the house first and outdoors last. Your dog can hold a settle they could not hold at 11 months.
18 to 24 months. The stage tails off. What you practised during the hard part is what you get back, which is the whole argument for doing something small every day.
These are patterns rather than promises. Your dog may run early or late on any of them. A quiet 10 month old dog is no more a training triumph than a loud one is a failure.
What does not happen
Nothing switches off at 12 months. A dog is not adult because they had a birthday, and food labels are not developmental advice.
Behaviour you never practised does not appear at the end. Adolescence going quiet gives you a calmer dog, not a trained one.
Nothing is permanently lost in the middle. Behaviour that is unavailable at the park is usually still available in your kitchen.
One thing to do today
Pick the single problem that costs you most this week, and work on that one alone for 2 weeks. Owners who take on 4 problems at once reach the end of adolescence with 4 half-trained skills.
Behaviour change is built from management plus reinforcement, and reward-based training has been associated with fewer reported problem behaviours. Both of those work better aimed at one skill than at four.
Safety note
Adolescence explains gradual change over weeks. It does not explain a change that arrived over days, or growling, snapping, biting, hiding, or a dog who has become frightened of ordinary things. Book a veterinary examination, because sudden behaviour change warrants one and pain often presents as a change in behaviour.
How to tell the stage is ending
The reliable signal is not calmness at home. It is your dog answering the first time in a busy place, on 4 or 5 tries out of 6.
Today's version, in one line: name the one problem you are working on, and leave the other three alone for 2 weeks.