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Is there a definitive response as to how long to continue searching? Actually there is an extremely simple one. No one but you knows how important your search is.
The simple response as to how long you should search? As long as you feel that you want or need to do so. A search is more significant for some people than others. However, most searchers that I have met are very intent on a successful result. You may decide that you need a break from searching or wish to give up entirely. The decision is yours to make, and no one else knows better than you what the right choice may be. Some people might tell you to give up, others will tell you to never give up. Do you is right for you, no one else can determine what is best for you.
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The amount of time that a search takes varies depending on how much time and effort is put into the search. It also may depend on how difficult a particular search might be. There are many factors that can affect the length of a search.
Most of the people that I know who are searching do not intend to give up; it is too important to them. They have an urgent need to know that nothing else can satisfy. Searching can require vast amounts of time and energy and become a nearly obsessive quest. Sometimes I worry that searchers postpone their lives while searching, but I know that may be inevitable to a degree. Although that might be fine for a time, eventually you may need to relegate less time and energy to your search and live your life. Search as long and hard as you need to, but do not let life pass you by in the meantime. It is easy to give advice in adoption, not nearly as easy to follow it!