Three FAA Inspectors came into a shop back 35 years ago with an owner and asked that same question: "How long should it take to do an Annual on this plane?"
The experienced mechanic had the correct answer: "That decision is made by the person responsible for the aircraft's airworthiness."
"
FAR 91.403
(a) The owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition, including compliance with
part 39 of this chapter."
The Mechanic went on and explained to the FAA that the owner 'delegated that responsibility to him' when he dropped off the keys, log books and said "do my Annual." It takes however long it takes to do the owner's responsibility. Put the aircraft into an Airworthy Condition.
As Owner/Operator of an aircraft you must know your responsibilities and follow through with them. If you choose not too, then you are relinquishing the responsibility to someone else.
Well, you can either relieve that person of that responsibility or let them finish.