The age a Great Dane/Labrador mix stops growing depends on which breed it takes after, but it is generally somewhere between 18 months and 3 years. Generally, the dog reaches its full height by 18 months but continues to fill out.
Smaller breeds generally reach their adult size more quickly than larger breeds. Medium to large breeds, including Labradors, reach their full height by 18 months but continue to fill out and gain weight until they are about 2 years old. Giant breeds such as Great Danes also reach most of their adult height by 18 months, but there are exceptions. However, they take longer to fill out to their full adult weight and may continue to gain weight until they are about 3 years old.