That really depends a great deal on the aircraft, the owner, and the mechanic or shop. A shiftless mechanic without scruples can do an annual in an hour and never touch the airplane. You don't want that. A decent mechanic working on a decent airplane that's been well maintained can take three days. The nature of the airplane and it's equipment and configuration, as well as any applicable AD's or things that need doing may prolong that and will certainly increase the cost.
Bear in mind that working on the airplane, fixing what's broken, etc, isnt part of the annual inspection. The inspection itself may be prolonged if things need attention on the way in to inspect, which is separate from the work done needed to fix what's found.
Many owners are unpleasantly surprised when their first inspection comes due, because a lot of things get found which they do not expect. Some get upset because the aircraft just received a prebuy inspection, and these things weren't found. Undone or unlogged AD's, service bulletins, or improper or unsigned maintenance can become real headaches, as can STC's or major repairs or alterations that are undocumented. The engine mount with a little pitting that you wouldn't give a second thought as a pilot may be found to require refurbishing by the mechanic, and there's nine hundred for the mount plus another thousand to remove and install the engine, and do whatever else is found in there...it adds up. Paperwork issues are minefields for a new owner that can quickly rack up the costs. Be prepared.
How long does it take? Until it's done.