Oil drains off the surfaces in engines in as little as 2 months, allowing corrosion to commence, which is aided by the amount of moisture in the air.
If the aircraft has been stored in a hot, arid environment, then the moisture from condensation problem is reduced, but never totally eliminated.
I have seen new, reconditioned engines, stored under cover, seize solid within 18 mths of being reconditioned, despite substantial coatings of oil on every surface when they were assembled.