... or ... ... or ... ...
I take the first version.
... Obviously this is the time which an absolute beginner will need.
Since the domain of A is (3, 20]
But: Since you never will reach A(t) = 3 with a final amount of t you'll never become an expert.
The state of proficiency is reached if A(t) = 5
... That means after 7½ weeks a worker has reached the state of a profi.