This is a great question and if anyone could provide a guarantee, or an educated guess, that would be impressive!!! I have been selling Real Estate for over 25 years, and have learned not to engage in such guesswork.
As the age old adage says " you make money when you buy real estate, but your realize it when you sell".
Before you make an investment for purely appreciation in a short period of time, contact a good local broker and discuss the transactional expenses that are involved with both the buy and the selling ends of the transaction.
By the way, there are very few co-op opportunities on the North Fork, and there are a fair bit more condos in the market.
Please keep in mind that for many buyers, there are returns that can not be measured in terms of dollars !! Enjoyment of the East End at your own schedule, relaxing and going to our many beaches and fine restaurants , or just relaxing in front of a fire and reading a good book !
Good Luck, and be sure to contact a knowledgable local broker !